brcmfmac: use brcmf_if::bssidx as index in interface list

Upon receiving an IF event from the firmware the interface
was created and stored on a list using the interface index.
With upcoming P2P feature the firmware will send a IF event
in which two interfaces have the same interface index. To
uniquely locate them on the list the bss index is now used.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>