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.

test/env_tables.c

@@ -130,13 +130,10 @@ uint8_t  lacpEnabled;
 uint8_t  lacp_actor_state[10];
 uint8_t  lacp_partner_state[10];
 uint8_t  lacp_rx_state[10];
-uint8_t  lacp_partner_sys[10][6];
+uint8_t  lacp_partner_sys[10][MAC_ADDR_LEN];
 uint16_t lacp_rx_count[10];
-uint8_t  lacp_port_lag[10] = {
-	LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE,
-	LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE,
-};
+uint8_t  lacp_port_lag[10];
 uint16_t lacp_lag_ports[LACP_NUM_LAGS];
-uint8_t  lacp_agg_sys[LACP_NUM_LAGS][6];
+uint8_t  lacp_agg_sys[LACP_NUM_LAGS][MAC_ADDR_LEN];
 uint8_t  lacp_agg_valid[LACP_NUM_LAGS];
 uint16_t lacp_members_last[LACP_NUM_LAGS];