net: gptp: Refactor the gPTP header addition

Increase the net_buf length in proper places. So when gPTP
header is added, increase the buf->len properly, and then
when gPTP packet type is added, increase it again properly.
This way the net_buf length is updated in more logical way.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>