net: ep93xx_eth stops receiving packets

Receiving small packet(s) in a fast pace leads to not receiving any
packets at all after some time.

After ethernet packet(s) arrived the receive descriptor is incremented
by the number of frames processed. If another packet arrives while
processing, this is processed in another call of ep93xx_rx. This
second call leads that too many receive descriptors getting released.

This fix increments, even in these case, the right number of processed
receive descriptors.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>