net: mscc: ocelot: remove unnecessary exposure of stats structures

Since commit 4d1d157f

 ("net: mscc: ocelot: share the common stat
definitions between all drivers") there is no longer a need to share the
stats structures to the world. Relocate these definitions to inside
ocelot_stats.c instead of a global include header.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>