rtl8192u: ieee80211_device: Replace semaphore wx_sem with mutex

The semaphore 'wx_sem' in ieee80211_device is a simple mutex,
so it should be written as one. Semaphores are going away in the future.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>