mwifiex: cfg80211: do not change virtual interface during scan processing

mwifiex: cfg80211: do not change virtual interface during scan processing

(1) Change virtual interface operation in cfg80211 process reset and
reinitilize private data structure.
(2) Scan result event processed in main process will dereference private
data structure concurrently, ocassionly crash the kernel.

The cornel case could be trigger by below steps:
(1) wpa_cli mlan0 scan
(2) ./hostapd mlan0.conf

Cfg80211 asynchronous scan procedure is not all the time operated
under rtnl lock, here we add the protect to serialize the cfg80211
scan and change_virtual interface operation.

Signed-off-by: Limin Zhu <liminzhu@marvell.com>
Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>