crc16: banked wrapper for cross-bank callers

The lookup tables live in BANK1, so the routine switches the code bank to
reach them with movc and never switches it back. That is the contract the
__naked declaration already implied: same bank or common segment only.
The name now says it.

crc16_bank1 is the routine unchanged. crc16 becomes a __banked wrapper in
BANK1, so a caller from another bank has its own bank put back by
__sdcc_banked_ret. The two existing call sites take the direct path and
pay nothing.

crc16.asm

@@ -3,6 +3,8 @@
 ;
 	.globl 	_crc_value
 	.globl 	_crc16
+	.globl 	_crc16_bank1
+	.globl 	__sdcc_banked_ret
 	.equ	BANK, 0x96
 ;	.equ	DPS, 0x86
 ; Variable in XMEM holding current CRC16 value, being updated
@@ -18,7 +20,7 @@ _crc_value::
 	.area HOME    (CODE)
 	.area CSEG    (CODE)
 ;	.area BANK1   (CODE)
-_crc16:
+_crc16_bank1:
 	mov	BANK, #1
 	push	dph
 	push	dpl
@@ -52,6 +54,10 @@ _crc16:
 
 	.area BANK1   (CODE)
 
+_crc16:
+	lcall	_crc16_bank1
+	ljmp	__sdcc_banked_ret
+
 crc16_table_l:
 	.byte #0x00, #0xc1, #0x81, #0x40, #0x01, #0xc0, #0x80, #0x41
 	.byte #0x01, #0xc0, #0x80, #0x41, #0x00, #0xc1, #0x81, #0x40