mwifiex: inform cfg80211 about disconnect if device is removed

If device is surprise removed, commands sent to FW including
deauthenticate command fail as bus writes fail.
We update our media_connected status to false and inform cfg80211
about disconnection only when command is successful. Since cfg80211
assumes device is still connected, it results into following
WARN_ON during unload:

WARNING: CPU: 0 PID: 18245 at net/wireless/core.c:937
         cfg80211_netdev_notifier_call+0x175/0x4d0 [cfg80211]()

Avoid this by emitting cfg80211_disconnected event even if the
deauthenticate command fails.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>