ANDROID: GKI: sound: usb: Add snd_usb_enable_audio_stream/find_snd_usb_substream

Add functions snd_usb_enable_audio_stream and find_snd_usb_substream
to resolve ABI diff.

Test: build
Bug: 151372815
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Change-Id: I658d525233be3eb657a3b36c81a3920e65e96a40
(cherry picked from commit f055b384)
[hridya: only include ABI diff, add EXPORT_SYMBOL_GPL,
add some pointer checks]
Signed-off-by: Hridya Valsaraju <hridya@google.com>