mmc_test: initialize mmc_test_lock statically

The mutex mmc_test_lock is initialized at every time mmc_test device
is probed. Probing another mmc_test device may break the mutex, if
the probe function is called while the mutex is locked.

This patch fixes it by statically initializing mmc_test_lock.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>