mwifiex: ensure user_scan_in not NULL while setting scan channel gap

Check for scan channel gap only when user_scan_in is not NULL.
user_scan_in is NULL for internal scans and if we check scan channel gap
at this place, it may result into crash.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>