sch_mqprio: Always set num_tc to 0 in mqprio_destroy()

All the cleanup code in mqprio_destroy() is currently conditional on
priv->qdiscs being non-null, but that condition should only apply to
the per-queue qdisc cleanup.  We should always set the number of
traffic classes back to 0 here.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>