wl1251: use ieee80211_rx_ni()

Because of SPI and SDIO wl1251 does everything in a workqueue, including
calling ieee80211_rx() which should be called with bottom halves disabled.
An error message is emitted because of this:

NOHZ: local_softirq_pending 08

Fix this by using ieee80211_rx_ni().

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