mwifiex: use del_timer_sync instead of del_timer

Use SMP safe del_timer_sync instead of del_timer for cancelling
timers.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>