USB: gadget: f_audio_source: Switch to DMA memory to support MMAP

- Music takes more time to resume on speaker after USB unplug.
- USB driver does not support the Non-blocking mode i.e.
  MMAP mode. When the USB is unplugged, write is blocked for
  10 sec, and after a 10 sec timeout, the USB accessory device
  is closed and the device switch to speaker occurs.
- Changes were made to implement the MMAP interface in the
  driver that is Non-blocking. Hence, the write thread gets
  unblocked immediately and the switch to speaker is
  instantaneous.

CRs-Fixed: 453134
Change-Id: I3020f56db31e78aed114e6a4021f519b1ed234c3
Signed-off-by: Deepa Madiregama <dmadireg@codeaurora.org>