mwifiex: use del_timer variant in interrupt context

We might be in interrupt context at few places. So replace
del_timer_sync() with del_timer(). This patch fixes a kernel
trace problem seen occasionally during our testing.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>