hdaa: Don't hold a mutex while creating child devices

[jhb]

Summary

The lock is already not held while deleting child devices, and the
bus_topo_lock is already held when child devices are created.

[jhb]

I can't see a reason to hold the lock in the function, it just calls device_add_child in a loop and is already under the bus_topo_lock. This also does clear up the witness warning I was getting on boot on my X1 Carbon after Warner's original change to use M_WAITOK.

[jhb]

Hmm, did you mean to approve this or request changes?

[imp]

silly button pushing nisse have gotten into my phone... Didn't mean to request changes.