media: zero reservedX fields in media_v2_topology

The MEDIA_IOC_G_TOPOLOGY implementation did not zero the reservedX fields.
Fix this.

Found with v4l2-compliance.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>