sound: usb: Enable autosuspend for usb audio device

This allows runtime suspend to kick in upon all the
children of usb device are suspended. It saves power
when usb audio device is connected but not data transfer
is taking place.

CRs-Fixed: 1003807
Change-Id: I227a74c029f10ffda9ceccc10322ad71e7fd0435
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>