CRAS: iodev - Limit min_cb_level

Limit min_cb_level to half the iodev's buffer size, this will
prevent the case that output device wake up while having no
enough room to write data.

BUG=chromium:477003
TEST=unittest

Change-Id: I8af95c55401c7e5e95b0479b09647dd090e3789d
Reviewed-on: https://chromium-review.googlesource.com/272366
Reviewed-by: Hsinyu Chao <hychao@chromium.org>
Commit-Queue: Hsinyu Chao <hychao@chromium.org>
Tested-by: Hsinyu Chao <hychao@chromium.org>
7 files changed