net: mscc: ocelot: fix mutex lock error during ethtool stats read

An ongoing workqueue populates the stats buffer. At the same time, a user
might query the statistics. While writing to the buffer is mutex-locked,
reading from the buffer wasn't. This could lead to buggy reads by ethtool.

This patch fixes the former blamed commit, but the bug was introduced in
the latter.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Fixes: 1e1caa97 ("ocelot: Clean up stats update deferred work")
Fixes: a556c76a ("net: mscc: Add initial Ocelot switch support")
Reported-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/all/20220210150451.416845-2-colin.foster@in-advantage.com/


Signed-off-by: Jakub Kicinski <kuba@kernel.org>