[BNX2]: Add bnx2_set_default_link().

Put existing code to setup the default link settings in this new
function.  This makes it easier to support the remote PHY feature in
the next few patches.

Also change ETHTOOL_ALL_FIBRE_SPEED to include 2500Mbps if supported.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>