qtnfmac: track broadcast domain of each interface

If firmware reports that it supports hardware switch capabilities,
driver needs to track and notify device whenever broadcast domain
of a particular network device changes (ie. whenever it's upper
master device changes).

Firmware needs a unique ID to tell broadcast domains from each other
which is an opaque number otherwise. For that purpose we can use
netspace:ifidx pair to uniquely identify each broadcast domain:
 - if netdev is not part of a bridge, then use it's own ifidx
   as a broadcast domain ID
 - if netdev is part of a bridge, then use bridge netdev ifidx
   as broadcast domain ID

Firmware makes sure that packets are only forwarded between
interfaces marked with the same broadcast domain ID.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>