staging: rtl8723bs: change pwrctrl lock to a mutex

This semaphore is used like a mutex, so it should use the regular
mutex API, as we do in the other copies of this driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>