saft: Fix the argument and the return value of get_internal_disk() The get_internal_disk() doesn't work on ARM platform. The argument should be a device with a partition and the return value should be a device without a partition. BUG=chrome-os-partner:13323 TEST=Run a FAFT test on ARM platform without error. Change-Id: Ib02df7a277bf17755d96370689255f2bf4a96d2f Reviewed-on: https://gerrit.chromium.org/gerrit/31850 Reviewed-by: Vic Yang <[email protected]> Commit-Ready: Tom Wai-Hong Tam <[email protected]> Tested-by: Tom Wai-Hong Tam <[email protected]>