brcmfmac: no interface combination check for single interface

The interface combinations are intended for use-case in which the driver
handles multiple interface concurrently. This means that the combinations
do not need to be checked when there is only a single interface active.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>