net: ftgmac100: Always register the MDIO bus when it exists

Both the Aspeed 2400 and 2500 and the original faraday version of the
MAC have MDIO bus controllers as part of the MAC. Since it exists,
always registering it makes the code simpler, and causes no harm. If
there is no mdio node in device tree, of_mdiobus_register() will fall
back to mdiobus_register(), making it safe.

AST2600 uses an external MDIO controller and does not have an embedded
MDIO bus in the MAC. For such configurations, the legacy MII probe path
must not be entered without a registered mii_bus.

Add an explicit check to fail gracefully when no MDIO bus is present,
preventing a NULL pointer dereference while keeping the intended
behavior for platforms without embedded MDIO.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>
Link: https://patch.msgid.link/20260206-ftgmac-cleanup-v5-9-ad28a9067ea7@aspeedtech.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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