net: mscc: ocelot: deny TX timestamping of non-PTP packets

It appears that Ocelot switches cannot timestamp non-PTP frames,
I tested this using the isochron program at:
https://github.com/vladimiroltean/tsn-scripts



with the result that the driver increments the ocelot_port->ts_id
counter as expected, puts it in the REW_OP, but the hardware seems to
not timestamp these packets at all, since no IRQ is emitted.

Therefore check whether we are sending PTP frames, and refuse to
populate REW_OP otherwise.

Fixes: 4e3b0468 ("net: mscc: PTP Hardware Clock (PHC) support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit fba01283)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
[Xulin: Original patch taken from
https://github.com/vladimiroltean/linux/commits/felix-dsa-tag-vlan-stable

]
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>