[media] tda10071: protect firmware command exec with mutex

There should be clearly some lock in order to make sure firmware
command in execution is not disturbed by another command. It has
worked as callbacks are serialized somehow pretty well and command
execution happens usually without any delays.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>