s390/qeth: remove check for packing mode in qeth_check_outbound_queue()

If qeth_check_outbound_queue() finds a partially filled TX buffer on
the queue and flushes it, then the queue _must_ have been in packing
mode.

Remove the redundant check when updating the relevant statistics.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>