[media] v4l: of: Return an int in v4l2_of_parse_endpoint()

When CONFIG_OF is not defined the v4l2_of_parse_endpoint() function is
defined as a stub that returns -ENOSYS. Make the real function return an
integer as well to be able to differentiate between the two cases.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>