The datasheet clearly states that all but a few registers can't be
modified when the device is running. Programming the intensity
parameters in the control set handler is thus prohibited. Program it
when starting the module instead.

This requires storing the intensity value internally as the module can
be started from the frame completion interrupt handler, and accessing
control values requires taking a mutex.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>