rtl8712: pwrctrl_priv: Replace semaphore lock with mutex

The semaphore 'lock' in 'pwrctrl_priv' is used as a simple mutex, so it
should be written as one. Semaphores are going away in the future.
_enter_pwrlock was using down_interruptible(), so the lock could be broken
by sending a signal. This could be a bug, because nothing checks the return
code here. Hence, using mutex_lock instead of the interruptible version.
Removing the now unused _enter_pwrlock and _down_sema.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>