staging: r8188eu: Use a Mutex instead of a binary Semaphore

Use a Mutex instead of a binary Semaphore for the purpose of enforcing
mutual exclusive access to the "pwrctrl_priv" structure.

Mutexes are sleeping locks similar to Semaphores with a 'count' of one
(like binary Semaphores), however they have a simpler interface, more
efficient performance, and additional constraints.

There is no change in the logic of the new code; however it is more
simple because it gets rid of four unnecessary wrappers:
_init_pwrlock(), _enter_pwrlock(),_exit_pwrlock(), _rtw_down_sema().

Actually, there is a change in the state in which the code waits for
acquiring locks, because it makes it in an uninterruptible state
(instead the old code used down_interruptibe()). Interruptible
waits are neither required nor wanted in this driver.

Tested with ASUSTek Computer, Inc. Realtek 8188EUS [USB-N10 Nano].

Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20211022171917.24363-1-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>