net: mscc: ocelot: fix hardware timestamp dequeue logic

net: mscc: ocelot: fix hardware timestamp dequeue logic

[ Upstream commit 5fd82200d870a5dd3e509c98ef2041f580b2c0e1 ]

The next hw timestamp should be snapshoot to the read registers
only once the current timestamp has been read.
If none of the pending skbs matches the current HW timestamp
just gracefully flush the available timestamp by reading it.

Signed-off-by: laurent brando <laurent.brando@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>