net: sched: taprio: Fix taprio_peek()

While traversing taprio's children qdisc list, if the gate is closed for
a given traffic class, we should continue traversing the list since the
remaining qdiscs may have skb ready for transmission.

This patch also takes this opportunity and changes the function to use
the TAPRIO_ALL_GATES_OPEN macro instead of the magic number '-1'.

Fixes: 5a781ccb (“tc: Add support for configuring the taprio scheduler”)
Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>