s390/qeth: fix packing buffer statistics

There's two spots in qeth_send_packet() where we don't accurately
account for transmitted packing buffers in qeth's performance
statistics:

1) when flushing the current buffer due to insufficient size,
   and the next buffer is not EMPTY, we need to account for that
   flushed buffer.
2) when synchronizing with the TX completion code, we reset
   flush_count and thus forget to account for any previously
   flushed buffers.

Reported-by: Nils Hoppmann <niho@de.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>