brcmfmac: extend struct brcmf_if with bssidx field

When the firmware notifies the driver about adding a new interface
it also provides an index for the bss associated with this interface.
This index will be needed for upcoming features like peer-to-peer.
By adding this index in struct brcmf_if it is easy to obtain as this
will be associated with the net_device private data.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>