acc_dispatcher: don't issue instruction from buffer if flushing

Instructions with side effects flush the unissued instructions from
the controller. The accelerator dispatcher buffer is flushed when
this happens and avoids accepting a new instruction, but was not
preventing the actual issue during a flush cycle.

core/acc_dispatcher.sv

@@ -257,7 +257,7 @@ module acc_dispatcher
       };
       // Wait until the instruction is no longer speculative.
       acc_req_valid      = insn_ready_q[acc_insn_queue_o.trans_id] ||
-                           (acc_commit && insn_pending_q[acc_commit_trans_id]);
+                           (acc_commit && insn_pending_q[acc_commit_trans_id] && !flush_unissued_instr_i);
       acc_insn_queue_pop = acc_req_valid && acc_req_ready;
     end
   end