rtl8192u: Replace semaphore scan_sem with mutex

The semaphore 'scan_sem' in rtl8192u 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>