util/flash_ec: Fix shellcheck warnings
util/flash_ec: Fix shellcheck warnings

PATCH_SIZE=$((${SPI_SIZE} - ${IMG_SIZE}))
              ^---------^ SC2004: $/${} is unnecessary on arithmetic variables.
			    ^---------^ SC2004: $/${} is unnecessary on
			                arithmetic variables.

let LOOP_COUNTER=LOOP_COUNTER-1
^-----------------------------^ SC2219: Instead of 'let expr', prefer
                                (( expr )) .

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

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I5f167427a4dad2ed8986e0b196042ba8d950eddb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3860069
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