usbnet: ipheth: do not stop RX on failing RX callback

RX callbacks can fail for multiple reasons:

* Payload too short
* Payload formatted incorrecly (e.g. bad NCM framing)
* Lack of memory

None of these should cause the driver to seize up.

Make such failures non-critical and continue processing further
incoming URBs.

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