hwmon: (s3c-hwmon) Use a real mutex

The semaphore which protects the ADC is semantically a mutex. Use a
real mutex.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>