mwifiex: abort scan while cancelling pending command

mwifiex_cancel_pending_ioctl() and
mwifiex_cancel_all_pending_cmd() are called in command timeout
and driver unload paths respectively.
If scan operation is in progress, we should abort it smoothly.

Reported-by: Tim Shepard <shep@alum.mit.edu>
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>