net: macb: Fix mdio child node detection

Commit 4d98bb0d ("net: macb: Use mdio child node for MDIO bus if it
exists") added code to detect if a 'mdio' child node exists to the macb
driver. Ths added code does, however, not actually check if the child node
exists, but if the parent node exists. This results in errors such as

macb 10090000.ethernet eth0: Could not attach PHY (-19)

if there is no 'mdio' child node. Fix the code to actually check for
the child node.

Fixes: 4d98bb0d ("net: macb: Use mdio child node for MDIO bus if it exists")
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
Tested-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20211026173950.353636-1-linux@roeck-us.net


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