net: dsa: lantiq_gswip: move MDIO bus registration to .setup()

net: dsa: lantiq_gswip: move MDIO bus registration to .setup()

Instead of registering the switch MDIO bus in the probe() function, move
the call to gswip_mdio() into the .setup() DSA switch op, so it can be
reused independently of the probe() function.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://patch.msgid.link/2650602042c0bfdc5664b88d59071ed4dca96c26.1756520811.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>