brcmfmac: kernel oops on wrong ifidx

When calling brcmf_rx_frame() ifidx might not get set and return non zero value.
Due to that ifp must not be set without proper ifidx otherwise it might get
a wrong pointer (since ifidx is not set correctly) causing ifp->ndev to access
an unmapped memory.

The fix initializes ifp to null and in case it doesn't get properly initialized
the skb will get deallocated in brcmu_pkt_buf_free_skb(skb) afterwards.

Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>