util/flash_ec: Fix shellcheck warning
util/flash_ec: Fix shellcheck warning

"(${FILTERED_CHIPS[*]}). Use --chip= to choose one."
  ^------------------^ SC2154: FILTERED_CHIPS is referenced but not
                               assigned.

The FILTERED_CHIPS variable was removed in commit
1641ef6a6e12ca4e26764ad0a083fc07a08da96d.

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

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I593431719a8a70e3e70d82be45b0461788d4c400
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3864976
Reviewed-by: Namyoon Woo <e8732cefb861f07768f47df788a8e8eb75efc09a@google.com>

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