CRAS: rstream: unsplit output and input shm

rstream has separate functions for output and input shm, but they
refer to the same pointer. Combine the two functions.

BUG=chromium:968724
TEST=unit tests, playback on device

Change-Id: Ib02acab1578cba7083006e01ce294d4d234bd969
Reviewed-on: https://chromium-review.googlesource.com/1649043
Commit-Ready: Fletcher Woodruff <fletcherw@chromium.org>
Tested-by: Fletcher Woodruff <fletcherw@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
7 files changed