brcmfmac: use bssidx from struct brcmf_if for bsscfg specific commands

brcmfmac: use bssidx from struct brcmf_if for bsscfg specific commands

The firmware interface has functions to send bsscfg specific commands
to the device. These functions currently have a bssidx parameter, but
that same information is stored in struct brcmf_if, which is in the
private data of the net_device parameter.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
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>