scripts/checkpatch: Fix a typo

scripts/checkpatch: Fix a typo

When running checkpatch.pl on a commit adding a file without
SPDX tag we get:

  Undefined subroutine &main::WARNING called at ./scripts/checkpatch.pl line 1694.

The WARNING level is reported by the WARN() method. Fix the typo.

Fixes: fa4d79c64da ("scripts: mandate that new files have SPDX-License-Identifier")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250303172508.93234-1-philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>