hal: return proper backend index for USB headset

Playback over USB headsets fails as the default backend index
is selected.

Return the correct backend index by adding a check for USB headset
to fix the issue.

Also backport a few bug fixes from USB tunnel mode testing on 8953.

CRs-Fixed: 1105780
Change-Id: If923f486a44070a4824b80199bcaed5d902462fe