media: amphion: Slightly simplify vpu_core_register()

"vpu_core->msg_buffer_size" is unused out-side of vpu_core_register().
There is no need to save this value in struct vpu_core.

Remove it and use a local variable instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>