net: mdio: mscc-miim: fix duplicate debugfs entry

This driver can have up to two regmaps. If the second one is registered
its debugfs entry will have the same name as the first one and the
following error will be printed:

[    3.833521] debugfs: Directory 'e200413c.mdio' with parent 'regmap' already present!

Give the second regmap a name to avoid this.

Fixes: a27a7628

 ("net: mdio: mscc-miim: convert to a regmap implementation")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220312224140.4173930-1-michael@walle.cc


Signed-off-by: Jakub Kicinski <kuba@kernel.org>