net: bfin_mac: Use phy_find_first() instead of open-coding it

Use phy_find_first() to find the first phy device instead of
open-coding it.

Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>