Two analog power planes for same voltage
Form what I read: 
"The digital ground pins (GND), the analog ground pins (AVSS), and the GND_CORE pins 
on the LAN8720 QFN are all connected internally to the exposed die paddle ground. "

So use a separate analog ground for your ADC as you mentioned above. Maybe it will work if both share the same ground plane, but it's easier to omit a ferrite if you don't need it than add one in if you do.