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