media: AV1: Make sure that bit depth in correctly initialize

Make sure that bit_depth field of V4L2_CTRL_TYPE_AV1_SEQUENCE
is initialized correctly before using it.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>