lacp: tidy up after review

Move port_bit() to rtl837x_common.h as PORT_BIT(), a uint16_t shift,
and MAC_ADDR_LEN from rtl837x_lldp.h next to it, and use MAC_ADDR_LEN
for every address in the LACP code instead of a literal 6. Define
lacp_mux_update() before its first caller and drop the forward
declaration, declare variables at the top of their blocks, put the
operator at the end of a continued line, drop the field value comments
of the LACPDU struct, and leave lacp_port_lag to lacp_init() instead of
initialising it in the test environment.

rtl837x_common.h

@@ -16,6 +16,8 @@
 #define TICKS_DUE(last, period)	((uint8_t)((uint8_t)ticks - (last)) >= (period))
 
 #define CPU_PORT        9
+#define MAC_ADDR_LEN    6
+#define PORT_BIT(p)     ((uint16_t)1 << (p))
 #define NUL			'\0'
 
 // Define Port-masks for 9-port devices and 6-port devices