staging: r8723au: pwrctrl_priv: Replace semaphore lock with mutex

The semaphore 'lock' in pwrctrl_priv is 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>