net: mscc: ocelot: strengthen type of "int i" in ocelot_stats.c

The "int i" used to index the struct ocelot_stat_layout array actually
has a specific type: enum ocelot_stat. Use it, so that the WARN()
comment from ocelot_prepare_stats_regions() makes more sense.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>