net: phy: Create phy_supported_speeds function which lists speeds currently...

net: phy: Create phy_supported_speeds function which lists speeds currently supported by a phydevice

phy_supported_speeds provides a means to get a list of all the speeds a
phy device currently supports.

Signed-off-by: Zach Brown <zach.brown@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>