rt2x00: Fix dead queue when skb allocation failed

When the RX skb allocation failed, we should recycle
the previously allocated skbuffer. By calling return
we would kill the RX queue completely since the
entry would be invalidated.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>