wl1271: Checking of rx descriptor status fixed

Fixed checking of status of the received packet. On wl1251 status is in the
flags field of the descriptor, on wl1271 there is a separate status field.

Signed-off-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>