airo: fix scan after SIOCSIWAP (airo_set_wap)

SIOCSIWAP (airo_set_wap) affects scan: only the AP specified by
SIOCSIWAP is present in scan results.

This makes NetworkManager work for the first time but then unable to
find any other APs.

Clear APList before starting scan and set it back after scan completes
to work-around the problem.

To avoid losing packets during scan, modify disable_MAC() to omit
netif_carrier_off() call when lock == 2.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>