message instead of relying on the fact that we are scheduled to send them.
The old method worked 99.9% of the time, but someone reported some periferals
that did MSG_REJECT at odd times (sometimes before we could send an SDTR
or WDTR) that we would construe as the response to an SDTR or WDTR message.
This also removes a possible race condition where after a bus reset (the
result of a command time out not during intial probe time), we might queue
two commands both requesting SDTR, WDTR or both.
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