During shutdown of TX or RX, the host may stop supplying or retrieving
sample data some time before a stop request causes the M0 to be set back
to idle mode.
This makes it common for a spurious shortfall to occur during shutdown,
giving the misleading impression that there has been a throughput
problem. In fact, the final shortfall is simply an artifact.
This commit detects when this happens, and excludes the spurious
shortfall from the stats.
To implement this, we back up the shortfall stats whenever a new
shortfall begins. If the new shortfall later turns out to be spurious,
as indicated by a transition to IDLE while it is ongoing, then we roll
back the stats to their previous values.
We actually only need to back up previous longest shortfall length. To
get a previous shortfall count, can simply to subtract one from the
current shortfall count.
This change adds four cycles to the two shortfall paths - a load and
store to back up the previous longest shortfall length.
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