mwifiex: reset scan_processing flag in failure cases

scan_processing flag should be reset when scan request is failed
due to some reasons Ex. memory allocation failure etc. Otherwise
further scan requests will be blocked.

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>