snd_hda: Add shutdown method

[tijl]

Summary

Power down the device on shutdown similar to what is done in the case
of suspend. The device may fail to attach on next boot without this.

PR: 286385

[christos]

In the PR we discussed putting the callout and taskqueue stuff before the bus_generic_detach() call in hdac_detach(). Should the same apply here to make it more robust?

[tijl]

Drain callout and taskqueue before shutting down child devices

[christos]

LGTM. We'll also need to MFC this to both stable/14 and releng/14.3.
CC @cperciva

[adrian]

nice catch!