[WATCHDOG] use mutex instead of semaphore in Berkshire USB-PC Watchdog driver

The Berkshire USB-PC Watchdog driver uses a semaphore as mutex.  Use the mutex
API instead of the (binary) semaphore.

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>