rtl8139: Do not consume the packet during overflow in standard mode.

When operation in standard mode, we currently return the size
of packet during buffer overflow.  This consumes the overflow
packet.  Return 0 instead so we can re-process the overflow packet
when we have room.

This fixes issues with lost/dropped fragments of large messages.

Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Message-id: 1441121206-6997-3-git-send-email-vyasevic@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>