smiapp: Take mutex during PLL update in sensor initialisation

commit f85698cd296f08218a7750f321e94607da128600 upstream.

The mutex does not serialise anything in this case but avoids a lockdep
warning from the control framework.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>