wl1271: use ieee80211_rx_ni()

Use the new ieee80211_rx_ni() function instead of ieee80211_rx().  Since we
use a workqueue to handle the RX path, we need to call the new function,
which disables bottom half handling.  This patch fixes the NOHZ:
local_softirq_pending messages.

CC: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>