gianfar: Use netdev_<level> when possible

Use a more current logging style.

Convert pr_<level> to netdev_<level> when a struct net_device is
available.  Add pr_fmt and neaten other formats too.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>