mwifiex: remove useless 'mwifiex_lock'

If mwifiex_shutdown_drv() is racing with another mwifiex_shutdown_drv(),
we *really* have problems. Kill the lock.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>