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>

include/linux/phy.h

@@ -667,6 +667,9 @@ struct phy_fixup {
 	int (*run)(struct phy_device *phydev);
 };
 
+const char *phy_speed_to_str(int speed);
+const char *phy_duplex_to_str(unsigned int duplex);
+
 /**
  * phy_read_mmd - Convenience function for reading a register
  * from an MMD on a given PHY.