scripts: ci: check_compliance: fix identity check for multiple DCOs

The current implementation of the identity check fails if multiple DCO
signoff lines are present and the first instance of the signoff line
does not belong to the author of the commit. This patch proposes a solution
that allows patches with multiple DCO signoff lines to pass the identity
check, regardless of the order of the signoff lines.

Signed-off-by: Mirai SHINJO <oss@mshinjo.com>