snd_hda: fix attaching to some AMD Radeon GPUs

HDMI audio did not work on my older Radeon GPU, the error message
during attach was:

hdac0: hdac_get_capabilities: Invalid corb size (0)

This is fixed by calling hdac_reset() before the HDA device
capabilities are retrieved. This solution was proposed by Andriy Gapon
in 2010, but apparently did not solve the problem reported at the time.

Approved by: christos
Differential Revision: https://reviews.freebsd.org/D50737