[media] v4l: rcar-fcp: Keep the coding style consistent

The Renesas multimedia drivers use ret to store return values, fix the
only exception in the rcar-fcp driver to keep the coding style
consistent.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>