net: davinci_emac: fix dev_err use at probe

Use platform device rather than net device in dev_err calls before net
device has been registered to avoid messages such as

	(null): DaVinci EMAC: Failed to get EMAC clock

Also replace remaining printks in probe with dev_{err,warn}.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>