PTP packets with 2-step TX timestamp requests are matched to packets
based on the egress port number and a 6-bit timestamp identifier.
All PTP timestamps are held in a common FIFO that is 128 entry deep.

This patch ensures that back-to-back timestamping requests cannot exceed
the hardware FIFO capacity. If that happens, simply send the packets
without requesting a TX timestamp to be taken (in the case of felix,
since the DSA API has a void return code in ds->ops->port_txtstamp) or
drop them (in the case of ocelot).

I've moved the ts_id_lock from a per-port basis to a per-switch basis,
because we need separate accounting for both numbers of PTP frames in
flight. And since we need locking to inc/dec the per-switch counter,
that also offers protection for the per-port counter and hence there is
no reason to have a per-port counter anymore.

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>