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.