mwifiex: move del_timer_sync(scan_delay_timer) call to fix memleak

Currently it is in mwifiex_adapter_cleanup() which doesn't get
called if driver initialization is failed causing memory leak.

scan_delay_timer is initialized in mwifiex_register(), so it
should be deleted in mwifiex_unregister(). Hence it has been
moved to appropriate place.

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>