port: let a pvid name an aggregation group

VLAN membership, PVID, egress tagging, isolation and MTU are all per
physical port in this ASIC, so nothing stopped a member of a working
aggregation group being given a different PVID from its peers. The group
then forwards asymmetrically depending on which member the hash picks,
and no part of the firmware says a word about it.

port_lag_of() answers which group a port belongs to, reading the
membership through the shared reader rather than a fourth private copy.
port_pvid_set() expands to the whole group when the port it is given is
a member, and ports outside a group keep the path they had.

This is the second and third of the three steps set out in #347. I said
there that the lookup would come when something needed it, which had it
the wrong way round: nothing in the tree asks which group a port is in,
so the lookup only earns its place alongside a caller. PVID is the
smallest such caller, and the rest of the per port settings can follow
the same shape once this one is agreed.

Built for SWTGW218AS and KP_9000_6XHML_X2 on sdcc 4.5.0.

rtl837x_port.h

@@ -62,6 +62,8 @@ void port_mirror_del(void) __banked;
 bool port_ingress_filter(__xdata uint8_t port, __xdata vlan_ingress_mode_t type) __banked;
 void port_l2_setup(void) __banked;
 uint16_t port_lag_members_get(uint8_t lag) __banked;
+#define PORT_LAG_NONE 0xff
+uint8_t port_lag_of(uint8_t port) __banked;
 void port_lag_members_set(__xdata uint8_t lag, __xdata uint16_t members) __banked;
 void port_lag_hash_set(__xdata uint8_t lag, __xdata uint8_t hash) __banked;
 void port_eee_enable_all(__xdata uint8_t speed) __banked;