[media] cec: split the timestamp into an rx and tx timestamp

When transmitting a message and waiting for a reply it would be good
to know the time between when the message was transmitted and when
the reply arrived. With only one timestamp field it was set to when
the reply arrived and the original transmit time was overwritten.

Just taking the timestamp in userspace right before CEC_TRANSMIT is
called is not reliable, since the actual transmit can be delayed if
the CEC bus is busy. Only the driver can fill this in accurately.

So split up the ts field into an rx_ts and a tx_ts. Also move the
status fields to after the 'reply' field: they were placed in a
strange position and make much more sense when grouped with the
other status-related fields.

This patch also makes sure that the timestamp is taken as soon as
possible.

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