net: mscc: ocelot: hide access to ocelot_stats_layout behind a helper

Some hardware instances of the ocelot driver support the MAC Merge
layer, which gives access to an extra preemptible MAC. This has
implications upon the statistics. There will be a stats layout when MM
isn't supported, and a different one when it is.
The ocelot_stats_layout() helper will return the correct one.
In preparation of that, refactor the existing code to use this helper.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>