of: constify of_property_check_flags() prop argument

This argument is not modified and thus can be set as const.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220601081801.348571-2-clement.leger@bootlin.com