[NET_SCHED]: Fix qdisc_restart return value when dequeue is empty

My previous patch that changed the return value of qdisc_restart
incorrectly made the case where dequeue returns empty continue
processing packets.

This patch is based on diagnosis and fix by Patrick McHardy.

Reported-and-debugged-by: Anant Nitya <kernel@prachanda.info>

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>