get rid of the mutex between access to the status block and myx_down

myx is unusual in that it has an explicit command to shut down the
chip that gets an interrupt when it's done. so myx_down sends the
command and has to sleep until it gets that interrupt. this moves
to using a single int to represent that state (so loads and stores
are atomic), and sleep_setup/sleep_finish in myx_down to wait for
it to change.

this has been running in production at work for a few months now
tested by chris@

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