mwifiex: minor cleanup and a fix in scan semaphore usage

mwifiex_request_scan() takes care of synchronous internal scan
performed by driver during association.
Currently the semaphore acquired for the scan is unnecessarily
released at the end of different paths. Also, failure paths
returning error code other than "-1" are not considered.

We will release it at the end of routine to fix above issues.

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>