[MTD] Use mutex instead of semaphore in dataflash driver

The MTD DataFlash 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>