rtl8188eu: 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.
_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.
Also, remove the now unused wrappers _init_pwrlock, _enter_pwrlock,
_exit_pwrlock and _rtw_down_sema.

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>