[prim,fpv] Waive impossible cover items from async alert fpv

[rswarbrick]

These case statement items have no effect on the design, but our style guide suggests including them to make the code more explicit. Unfortunately, Jasper automatically infers cover items for them. Get rid of them.

[rswarbrick]

With these waivers, running the testbench with util/dvsim/dvsim.py --gui hw/top_earlgrey/formal/top_earlgrey_fpv_prim_cfgs.hjson --select-cfg=prim_alert_rxtx_async_fpv gives the following coverage report:

The remaining coverage holes are genuine, such as this one:

(the default case of a 7-state FSM).

But those need addressing with a separate change.