dsa: Use netdev_<level> instead of printk

Neaten and standardize the logging output.

Other miscellanea:

o Use pr_notice_once instead of a guard flag.
o Convert existing pr_<level> uses too.

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