rtlwifi: fix memory leak for USB device

Free skb for received frames with a wrong checksum. This can happen
pretty rapidly, exhausting all memory.

This fixes a memleak (detected with kmemleak). Originally found while
using monitor mode, but it also appears during managed mode (once the
link is up).

Cc: stable@vger.kernel.org
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
ACKed-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>