util/flash_ec: Disable shellcheck warnings
util/flash_ec: Disable shellcheck warnings

These shellcheck warnings in this script don't appear to cause problems
and don't have easy fixes, so we'll disable them so that flash_ec is
free of any shellcheck warnings.

BRANCH=none
BUG=b:242127759
TEST=./util/flash_ec --board=dartmonkey

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: Ie5db0b19f254c41983d21c7a985bf2cd98d8140d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3864979
Reviewed-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec