CRAS: iodev - Skip update_channel_layout for 2-channel output.

If the requested channel count when opening an output device <= 2
then we can skip the update channel layout call and reduce the time
spent to init a device.

BUG=chromium:719812
TEST=Tested on reef.

Change-Id: I46cba552751440a2043b0778b8b22a51a6113e3e
Reviewed-on: https://chromium-review.googlesource.com/499910
Commit-Ready: Chinyue Chen <chinyue@chromium.org>
Tested-by: Chinyue Chen <chinyue@chromium.org>
Reviewed-by: Hsinyu Chao <hychao@chromium.org>
1 file changed