Always send STANDBY IMMEDIATE when shutting down

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Summary

To save SMART data and for a drive to understand that it's been nicely
shutdown, we need to send a STANDBY IMMEDIATE. Modify adaspindown to
use a local CCB on the stack. When we're panicing, used
xpt_polled_action rather than cam_periph_runccb so that we can SEND
IMMEDIATE after we've shutdown the scheduler.

Sponsored by: Netflix

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sys/cam/ata/ata_da.c

3519

This should be adaerror()

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Will fix the cosmetics before push.