arch/risc-v/src/mpfs/mpfs_ethernet.c: Set correct return value

Return error in case of mpfs_i2cbus_initialize() failure.

Signed-off-by: Jani Paalijarvi <jani.paalijarvi@unikie.com>

arch/risc-v/src/mpfs/mpfs_ethernet.c

@@ -3357,6 +3357,10 @@ static int mpfs_phyinit(struct mpfs_ethmac_s *priv)
     {
       ret = ksz9477_i2c_init(bus, KSZ9477_PORT_SGMII);
     }
+  else
+    {
+      ret = -EINVAL;
+    }
 
 #endif