wl1251: fix event handling mechanism

In case both A and B events occured simultaneously, current code
would only process A and clear both interrupts. Make it process both
events instead.

Based on wl1271 patches by Juuso Oikarinen:
1fd2794f
13f2dc52

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>