fpsensor: Fix g++ compiler error
fpsensor: Fix g++ compiler error

When compiling the host tests, g++ warns about the following:

common/fpsensor/fpsensor.cc:491:25: error: comparison of integer
expressions of different signedness: 'uint32_t' {aka 'unsigned int'} and
'int8_t' {aka 'signed char'} [-Werror=sign-compare]
  491 |                 if (fgr == template_newly_enrolled) {
      |                     ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~

BRANCH=none
BUG=b:234181908, b:244781166
TEST=make host-fpsensor_crypto
TEST=./util/compare_build.sh -b fp -j 120
     => MATCH

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: Ida17f6c123f6d73589774dd16ba60d0d54a8ee17
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3689786
Reviewed-by: Bobby Casey <a3c107f0b4efaee07148ab0813ba992bba562a69@google.com>
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