[NET_SCHED]: Revert "HTB: fix incorrect use of RB_EMPTY_NODE"

With commit 10fd48f2 [1] ,  RB_EMPTY_NODE
changed behaviour so it returns true when the node is empty as expected.
Hence Patrick McHardy's fix for sched_htb.c should be reverted.

Signed-off-by: Ismail Donmez <ismail@pardus.org.tr>
ACKed-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>