[media] tw9910: Don't access the device in the g_mbus_fmt operation

The g_mbus_fmt operation only needs to return the current mbus frame
format and doesn't need to configure the hardware to do so. Fix it to
avoid requiring the chip to be powered on when calling the operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>