sfp: read the EEPROM in blocks instead of a byte at a time

The I2C controller transfers up to sixteen bytes per transaction and
page_impl.c already used that for sfp_send_data(), while sfp_read_reg()
asked for one byte and every caller looped. Reading a module therefore
cost one address phase per byte: 87 transactions when a module is
inserted, 52 for the sfp command, 36 for the vendor block in status.json.

sfp_read_block() replaces sfp_read_reg() and the callers that already
wanted a run of registers ask for it once: the vendor fields as three
16 byte pages, the diagnostics as one transfer, rate and encoding
together. That drops the three paths above to 8, 6 and 3 transactions,
and sfp_send_data() loses its copy of the transfer.

The vendor loops now run over 16..63 rather than 20..59 so the page base
is a multiple of sixteen and the index into the buffer is a single AND.
The four extra bytes at each end are read and discarded. The diagnostics
read asks for 16 bytes rather than the 15 it uses, because 16 is a width
the shipped firmware already exercises and 15 is not.

The device address, the bus selection and the start bit go into the
control register in one write now that the memory address is written
first, so a transfer touches that register once instead of three times.
The register reads take their result from the SFRs directly rather than
through the sfr_data mirror. The result is a bool and the destination is
sfp_buf, so a caller that cares about a failed transfer looks at the
return value instead of a flag.

Every caller gives up on the first failed read rather than carrying a
flag to the end, which is why the module read moved out of handle_sfp
into a function of its own. A module whose read fails is left marked as
absent, so the next poll retries it instead of configuring the SerDes
from bytes that never arrived.

BANK1 -194 bytes, BANK2 +382, common segment +44, xdata +15 for the
buffer, and one byte more of internal RAM free than before the series.
Built for all 25 machine definitions on sdcc 4.5.0; the tightest common
segment is 98 bytes free on SWTG024AS_V2_0, against 54 before this
series.

rtl837x_common.h

@@ -149,8 +149,8 @@ void sleep(uint16_t t);
 void write_char_no_syslog(char c);
 void write_char(char c);
 void print_reg(uint16_t reg);
-uint8_t sfp_read_reg(uint8_t slot, uint8_t reg) __banked;
-extern __xdata uint8_t sfp_i2c_fail;
+bool sfp_read_block(uint8_t slot, uint8_t reg, uint8_t len) __banked __reentrant;
+extern __xdata uint8_t sfp_buf[16];
 void reg_bit_set(uint16_t reg_addr, char bit);
 void reg_bit_clear(uint16_t reg_addr, char bit);
 uint8_t reg_bit_test(uint16_t reg_addr, char bit);
@@ -169,7 +169,7 @@ void tcpip_output(void);
 uint8_t read_flash(uint8_t bank, __code uint8_t *addr);
 void get_random_32(void);
 void read_reg_timer(__xdata uint32_t * tmr);
-void sfp_print_info(uint8_t sfp);
+bool sfp_print_info(uint8_t sfp);
 bool gpio_pin_test(uint8_t pin);
 void set_sys_led_state(uint8_t state);
 void sds_read(uint8_t sds_id, uint8_t page, uint8_t reg);