cappuccino/rf: reset wb_hazard_a/b instead of wb_hazard_result

The value of wb_hazard_result isn't used until wb_hazard_a/b
is asserted, so it makes much more sense to set those
to a known value (0) at reset than resetting the result signal.
cappuccino/fetch: stop flushing on du_restart_i
cappuccino/ctrl: clear branch_step when npc is written

All gathered information about branches and delay slots
are invalid when the user writes npc, so this information
should be cleared when npc has been written.