net: mdio: Check regmap pointer returned by device_node_to_regmap()

net: mdio: Check regmap pointer returned by device_node_to_regmap()

The call to device_node_to_regmap() in airoha_mdio_probe() can return
an ERR_PTR() if regmap initialization fails. Currently, the driver
stores the pointer without validation, which could lead to a crash
if it is later dereferenced.

Add an IS_ERR() check and return the corresponding error code to make
the probe path more robust.

Fixes: 67e3ba978361 ("net: mdio: Add MDIO bus controller for Airoha AN7583")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251031161607.58581-1-alok.a.tiwari@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>