flash_ec: Fix SERVO_TYPE test
flash_ec: Fix SERVO_TYPE test

Without this, we see this line when running flash_ec.
util/flash_ec: line 787: [: =~: binary operator expected

These kind of tests require double brackets.

BUG=b:77825616
BRANCH=none
TEST=run util/flash_ec using servo_micro on staff.

Change-Id: I6baecec2252276ac06992fd2b2e50f74d55805f2
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1018560
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

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