net: dsa: felix: fix race between reading PSFP stats and port stats

net: dsa: felix: fix race between reading PSFP stats and port stats

Both PSFP stats and the port stats read by ocelot_check_stats_work() are
indirectly read through the same mechanism - write to STAT_CFG:STAT_VIEW,
read from SYS:STAT:CNT[n].

It's just that for port stats, we write STAT_VIEW with the index of the
port, and for PSFP stats, we write STAT_VIEW with the filter index.

So if we allow them to run concurrently, ocelot_check_stats_work() may
change the view from vsc9959_psfp_counters_get(), and vice versa.

Fixes: fa5916fb5652 ("net: dsa: felix: support psfp filter on vsc9959")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220629183007.3808130-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>