[media] solo6x10: use v4l2_get_timestamp to fill in buffer timestamp

The timestamp of a v4l2_buffer was advertised as being CLOCK_MONOTONIC,
but instead a timestamp from a header field was used. This is inconsistent
and not what applications expect. Use v4l2_get_timestamp to properly
set the timestamp.

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