usbnet: ipheth: drop RX URBs with no payload

On iPhone 15 Pro Max one can observe periodic URBs with no payload
on the "bulk in" (RX) endpoint. These don't seem to do anything
meaningful. Reproduced on iOS 17.5.1 and 17.6.

This behaviour isn't observed on iPhone 11 on the same iOS version. The
nature of these zero-length URBs is so far unknown.

Drop RX URBs with no payload.

Signed-off-by: Foster Snowhill <forst@pen.gy>
Signed-off-by: David S. Miller <davem@davemloft.net>