net: phy: display human readable PHY speed settings

Use a convenience function: phy_speed_to_str() which will display human
readable speeds.

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