net_sched: red: Avoid devision by zero

Do not allow delta value to be zero since it is used as a divisor.

Fixes: 8af2a218 ("sch_red: Adaptative RED AQM")
Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>