wl1251: use __dev_alloc_skb() on RX

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

Based on a patch for wl1271 by Luis R. Rodriguez.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>