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_lldp.h

@@ -7,7 +7,6 @@
 #define LLDP_ETHERTYPE       0x88cc
 #define LLDP_ETHERTYPE_LENGTH 2
 #define LLDP_MAX_FRAME       256
-#define MAC_ADDR_LEN    6
 #define LLDP_TX_INTERVAL_TICKS 6000
 
 #define LLDP_PORT_ID_TLV_TYPE 2