net: dsa: mt7530: return mt7530_setup_mdio & mt7531_setup_common on error

The mt7530_setup_mdio() and mt7531_setup_common() functions should be
checked for errors. Return if the functions return a non-zero value.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>