brcmfmac: consolidate ifp lookup in driver core

In rx path the firmware provide an interface index which is used to
map to a struct brcmf_if instance. However, this involves some trick
that is done in two places. This is changed by having driver core
providing brcmf_get_ifp() function.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@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>