net: dsa: microchip: fill regmap_config name

Use the register value width as the regmap_config name to prevent the
following error when the second and third regmap_configs are
initialized.
 "debugfs: Directory '${bus-id}' with parent 'regmap' already present!"

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>