[NET_SCHED]: prio qdisc boundary condition

This fixes an out-of-boundary condition when the classified
band equals q->bands. Caught by Alexey

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>