mlxsw: reg: Add lag_vid field to SFD register

Unicast LAG records in the Switch Filtering Database (SFD) register have
a lag_vid field indicating the VLAN ID in case of vFIDs. This field is
no longer reserved since we are going to add support for VLAN devices on
top of LAG.

Add the lag_vid field to be used by VLAN devies on top of LAG.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>