media: vimc: keep the error value when adding an entity fails

Currently when the 'add' callback of an entity fails, a
NULL is returned. This hides the error code of the failure
and always returns -EINVAL.

Replace return NULL with return ERR_PTR(ret) to improve debugging.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>