qeth: Fix default queue setting in priority queueing

The default queue for queueing modes prio_queueing_prec and
prio_queueing_tos is supposed to be QETH_DEFAULT_QUEUE. However, neither
mode will reset the default default_out_queue value when enabled, leaving
a previously set value activate as the default.
E.g. enabling one of the fixed queueing modes, e.g. no_prio_queueing:0,
and successively switching to any of the prio_queueing_* modes will
leave default_out_queue at the previous (wrong) value 0.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>