net: mscc: ocelot: add missed parentheses around macro argument

Commit 2f187bfa ("net: ethernet: ocelot: remove the need for num_stats
initializer") added a macro that patchwork warned it lacked parentheses
around an argument. Correct this mistake.

Fixes: 2f187bfa

 ("net: ethernet: ocelot: remove the need for num_stats initializer")
Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>