[media] omap3isp: Don't access ISP_CTRL directly in the statistics modules

Use the existing omap3isp_subclk_enable() and omap3isp_subclk_disable()
functions instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>