media: vimc: cleanup code that assigns entity in entities array

Since the add callback returns NULL on failure and the array
is initialized to NULLs, there is no need for the intermediate
assignment to local var.

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