brcmfmac: pass ifp pointer in brcmf_fws_find_mac_desc()

Instead of passing the ifidx and lookup the ifp inside the
function brcmf_fws_find_mac_desc() simply pass the ifp as
parameter.

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