core/cortex-m: Invert if statement for readability
core/cortex-m: Invert if statement for readability

BRANCH=none
BUG=b:172020503
TEST=./util/compare_build.sh -b all -j 120
     => MATCH

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I39f9d35b22d25c923a768bf86b3a16413b9f0a4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3219871
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

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