brcmfmac: check ndev pointer

Clang static analysis reports this error

brcmfmac/core.c:490:4: warning: Dereference of null pointer
        (*ifp)->ndev->stats.rx_errors++;
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In this block of code

	if (ret || !(*ifp) || !(*ifp)->ndev) {
		if (ret != -ENODATA && *ifp)
			(*ifp)->ndev->stats.rx_errors++;
		brcmu_pkt_buf_free_skb(skb);
		return -ENODATA;
	}

(*ifp)->ndev being NULL is caught as an error
But then it is used to report the error.

So add a check before using it.

Fixes: 91b63280

 ("brcmfmac: Use net_device_stats from struct net_device")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200802161804.6126-1-trix@redhat.com