net: mscc: ocelot: fix ineffective WARN_ON() in ocelot_stats.c

Since it is hopefully now clear that, since "last" and "layout[i].reg"
are enum types and not addresses, the existing WARN_ON() is ineffective
in checking that the _addresses_ are sorted in the proper order.

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>