netdev: octeon_mgmt: Add support for 1Gig ports.

The original hardware only supported 10M and 100M.  Later versions
added 1G support.  Here we update the driver to make use of this.

Also minor logic clean-ups for testing PHY registration error codes
and TX complete high water marks.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: David S. Miller <davem@davemloft.net>