s390/qeth: set TX IRQ marker on last buffer in a group

When qeth_flush_buffers() gets called for a group of TX buffers
(currently up to 2 for OSA-style devices), the code iterates over each
buffer for some final processing.

During this processing, it sets the TX IRQ marker on the leading buffer
rather than the last one. This can result in delayed TX completion of
the trailing buffers. So pull the IRQ marker code out of the loop, and
apply it to the final buffer.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>