net: mscc: ocelot: strengthen type of "u32 reg" and "u32 base" in ocelot_stats.c

Use the specific enum ocelot_reg to make it clear that the region
registers are encoded and not plain addresses.

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>