HHF qdisc: fix jiffies-time conversion.

This is to be compatible with the use of "get_time" (i.e. default
time unit in us) in iproute2 patch for HHF as requested by Stephen.

Signed-off-by: Terry Lam <vtlam@google.com>
Acked-by: Nandita Dukkipati <nanditad@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>