wl1271: use __dev_alloc_skb() on RX

RX is handled in a workqueue therefore allocating for GFP_ATOMIC
is overkill and not required.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>