wl1271: don't get received frames from hardware in PLT mode

Most probably patch "wl1271: add most of the normal initialization commands
to PLT mode" enabled the RX path in firmware so that now driver received
frames and passed them to mac80211, which warned about them.

Workaround this by not retrieving frames from the hardware, just ignore them.

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