net/fsl: quieten expected MDIO access failures

MDIO reads can happen during PHY probing, and printing an error with
dev_err can result in a large number of error messages during device
probe.  On a platform with a serial console this can result in
excessively long boot times in a way that looks like an infinite loop
when multiple busses are present.  Since 0f183fd1 (net/fsl: enable
extended scanning in xgmac_mdio) we perform more scanning so there are
potentially more failures.

Reduce the logging level to dev_dbg which is consistent with the
Freescale enetc driver.

Cc: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Jamie Iles <jamie@nuviainc.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/xgmac_mdio.c

@@ -229,7 +229,7 @@ static int xgmac_mdio_read(struct mii_bus *bus, int phy_id, int regnum)
 	/* Return all Fs if nothing was there */
 	if ((xgmac_read32(&regs->mdio_stat, endian) & MDIO_STAT_RD_ER) &&
 	    !priv->has_a011043) {
-		dev_err(&bus->dev,
+		dev_dbg(&bus->dev,
 			"Error while reading PHY%d reg at %d.%hhu\n",
 			phy_id, dev_addr, regnum);
 		return 0xffff;