net: mscc: ocelot: fix race between ndo_get_stats64 and ocelot_check_stats_work

The 2 methods can run concurrently, and one will change the window of
counters (SYS_STAT_CFG_STAT_VIEW) that the other sees. The fix is
similar to what commit 7fbf6795 ("net: mscc: ocelot: fix mutex lock
error during ethtool stats read") has done for ethtool -S.

Fixes: a556c76a ("net: mscc: Add initial Ocelot switch support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>