Fix shellcheck errors in touch-update-legacy.sh

Updated code to match recommended shell code best practices.

BUG=b:259595539
TEST=CQ
TEST=Deployed on Starmie and used to downgrade+upgrade touchscreen \
       firmware

Change-Id: I461248a57e0ea2aea502338ac76ab02bdea86472
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/touch_updater/+/4953936
Reviewed-by: Kenneth Albanowski <kenalba@google.com>
Tested-by: Henry Barnor <hbarnor@chromium.org>
Reviewed-by: Torsha Banerjee <torsha@google.com>
Commit-Queue: Henry Barnor <hbarnor@chromium.org>
1 file changed