net: mvneta - Fixed recv() when multiple packets have arrived.

This patch fixes a problem in the mvneta driver where if more than
one packet arrives between calls to mvneta_recv(), the additional
descriptors will be marked as free even though only one descriptor
has been read and processed from the receive queue.  This causes
the additional packet(s) to be delayed until the next packet arrives.
>From this point on all packets will be delayed because the receive
queue will contain unprocessed packets but the hardware shows no
busy descriptors.

Signed-off-by: Jason Brown <jason.brown@apcon.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>