net: phy: split out PHY speed and duplex string generation

Other code would like to make use of this, so make the speed and duplex
string generation visible, and place it in a separate file.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>