net: phy: sftp: print debug message with text, not numbers

Convert the state numbers, device state, etc from numbers to strings
when printing debug messages.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>