net: dp83822: Fix OF_MDIO config check

When CONFIG_OF_MDIO is set to be a module the code block is not
compiled. Use the IS_ENABLED macro that checks for both built in as
well as module.

Fixes: 5dc39fd5 ("net: phy: DP83822: Add ability to advertise Fiber connection")
Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250423044724.1284492-1-johannes.schneider@leica-geosystems.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>