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: NBinoy Jayan <binoy.jayan@linaro.org>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>