USB audio gadget: Prefix all macro definitions with UAC_ in linux/usb/audio.h

linux/usb/audio.h is a public header file that includes definitions
exported to userspace. To avoid namespace clashes, prefix all macro
definitions with UAC_. Existing macros and structures prefixed with
USB_AC_ and USB_AS_ are renamed for consistency.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>