usbaudio: Make the USB audio HAL actually work.

 * Set a default device if none is set.
 * Shush some logging.
 * Only use the mmap interface on QCOM devices.

Change-Id: I790a6733e85b169e645e020fa92608a0d4fea3c2
Merge "usbaudio: Make the USB audio HAL actually work." into jb4.3