Fix shellcheck errors in chromeos-touch-common.sh

Updated code to match recommended shell code best practices.

BUG=b:259595539
TEST=CQ
TEST=Deployed on Starmie, Zydron and used to downgrade+upgrade
     touch firmware.

Change-Id: I22eedc67fe1919f3da31db76f1771bf07a0affed
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/touch_updater/+/5018303
Commit-Queue: Henry Barnor <hbarnor@chromium.org>
Reviewed-by: Kenneth Albanowski <kenalba@google.com>
Tested-by: Henry Barnor <hbarnor@chromium.org>
3 files changed