media: stm32-dcmi: simplify of_node_put usage

This does not fix any bug - this is just a code simplification. As
np is not used after passing it to v4l2_fwnode_endpoint_parse() its
refcount can be decremented immediately and at one location.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>