net: gptp: Invert priority of outgoing packets

The previous implementation was setting priority 3 (Critical
applications) for Sync, Pdelay and Pdelay_Resp messages; and priority 6
(Internetwork control) for the other PTP messages. This might have been
done under the incorrect understanding that "Critical applications" was
the highest priority.
This resulted in the Pdelay_Resp_Follow_Up getting sent before the
Pdelay_Resp message when enough TC_TX queues were being used
(NET_TC_TX_COUNT >= 2), which is an inversion compared to the intended
order.

Invert the priority of the outgoing PTP packets so that the PTP event
messages are sent with higher priority, as was originally intended.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>