cam: Add a number of asserts to catch bad transactions

[imp]

Summary

Ensure that we're in the right state / priority for each of the states
in the driver. These asserts assured that a prior patch that I committed
to fix a priority leak worked when a drive departed (and bounced back
too!). These have been running in our production since I committed the
change and haven't trigged.

Sponsored by: Netflix

[imp]

sys/cam/ata/ata_da.c

2347

I was torn on this one. Maybe it should be an assert instead.
However, over the past couple of years since I pushed the fix for this bug, these messages have not appeared in our fleet (before, we'd see a bunch a week, some subset of which would lead to a badly lost race that panic'd the machine).

[imp]

add da