spill_register_flushable assertion improvement

[tsyw]

When downstream has backpressure, the flush function cannot work properly. so can we add the following sva property?
assupe property (
@(posedge clk_i) disable iff (!rst_ni)
flush_i |-> ready_i
) else $warning("Trying to flush when downstream not ready");

[tsyw]

sorry for the typo:
assume property ( @(posedge clk_i) disable iff (!rst_ni) flush_i |-> ready_i ) else $warning("Trying to flush when downstream not ready");