sch_prio: Use return value from inner qdisc requeue

Use return value from inner qdisc requeue when value returned isn't
NET_XMIT_SUCCESS, instead of always returning NET_XMIT_DROP.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>