net: phy: broadcom: Add support code for downshift/Wirespeed

Broadcom's Wirespeed feature allows us to configure how auto-negotiation
should behave with fewer working pairs of wires on a cable. Add support
code for retrieving and setting such downshift counters using the
recently added ethtool downshift tunables.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>