s390/qeth: fix cancelling of TX timer on dev_close()

With the introduction of TX coalescing, .ndo_start_xmit now potentially
starts the TX completion timer. So only kill the timer _after_ TX has
been disabled.

Fixes: ee1e52d1

 ("s390/qeth: add TX IRQ coalescing support for IQD devices")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>