rtlwifi: rtl_pci: Fix problem of too small skb->len

In commit 8020919a

 ("mac80211: Properly handle SKB with radiotap
only"), buffers whose length is too short cause a WARN_ON(1) to be
executed. This change exposed a fault in rtlwifi drivers, which is fixed
by regarding packets with skb->len <= FCS_LEN as though they are in error
and dropping them. The test is now annotated as likely.

Cc: Stable <stable@vger.kernel.org> # v5.0+
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>