fpsensor: Tidy up fp_match_success
fpsensor: Tidy up fp_match_success

Simplify logic and replace #ifdef with the compiler hint that
the function might not be used in certain builds (e.g. tests).

BRANCH=none
BUG=b:222592305
TEST=make buildall -j

Signed-off-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>
Change-Id: I8091cd2bd8e2770d9aa62a80a67d654ddfea1c75
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3508717
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

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