brcmfmac: use brcmf_get_ifp() to map ifidx to struct brcmf_if instance

The knowledge on how to map the interface index to a struct brcmf_if
instance is in brcmf_get_ifp() so use that function when only the
interface index is known instead of accessing brcmf_pub::iflist
directly.

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>