[NET_SCHED]: HFSC: fix thinko in hfsc_adjust_levels()

When deleting the last child the level of a class should drop to zero.

Noticed by Andreas Mueller <andreas@stapelspeicher.org>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>