mwifiex: abort scan upon interface down

When the interface is down, we will abort scan by calling
cfg80211_scan_done() with abort option. This fixes WARN_ON
triggered by cfg80211 in wdev_cleanup_work().

Driver's internal variables/flags are cleared once we get
response for current scan command. Meanwhile we will block
new scan request from cfg80211.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>