Butterfly / Stout: Change the return code of 'mosys storage set phy_speed'.

'mosys storage set phy_speed' is used for SanDisk-U100 PHY tuning.
We add it into factory test but it shows failure for non-SanDisk-U100 SSDs.
Fix the failure by changing the default return code to 0.

TEST=Manual.
     1. "mosys storage set phy_speed SATA3 -vvv" commands on Butterfly.
     2. "echo $?", it should show 0.
BUG=chrome-os-partner:18647
BRANCH=butterfly, stout.

Change-Id: I62f0ab8cd2f18be1de9bd6e52150d06651b8a6a6
Reviewed-on: https://gerrit.chromium.org/gerrit/47556
Tested-by: Bowgo Tsai <bowgotsai@chromium.org>
Reviewed-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Commit-Queue: Bowgo Tsai <bowgotsai@chromium.org>
3 files changed