net: dnet: Use phy_find_first() helper

Replace the open coded search for the first phy with a call to the
existing helper function.

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