checkpatch.pl: Add additional granularity to malformed commit errors

Commit-Ids are expected to be 12 to 40 characters, followed by a
description enclosed by parenthesis. The current error message only
mentions the length requirement, causing confusion when a commit-Id is
formatted properly but the description is not inside parenthesis.

Change-Id: If4c5a9c197146de9ada7c618a1cfff38514e611c
Signed-off-by: Ian Maund <imaund@codeaurora.org>