[media] v4l: omap4iss: Enable driver compilation as a module

Now that the driver doesn't use the non-exported omap4_ctrl_pad_readl
and omap4_ctrl_pad_writel functions nothing prevents it from being
compiled as a module anymore.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>