mwifiex: Replace semaphore async_sem with mutex

The semaphore 'async_sem' is used as a simple mutex, so
it should be written as one. Semaphores are going away in the future.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>