rtl8xxxu: Adjust RX skb size to include space for phystats

The old allocation didn't leave space for phystats in the buffer,
allowing the packet to be rejected if a frame size of size
IEEE80211_MAX_FRAME_LEN was received.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>