cam: Enforce real priorities in xpt_action for queued ccbs.

All queued CCBs should be created with a real priority (one that's not
CAM_PRIORITY_NONE). Recently, I introduced a bug that revealed a latent
MMC bug where it would stop enumerating due to a bad priority. Add an
assert to catch that (the other bug in mmc_da that it found has been
fixed).

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