checkpatch: relax check for length of git commit IDs

Checkpatch currently warns if a git commit ID (in the changelog,
usually) is less than 12 characters or more than 16.  The "more than 16"
is excessive.  Change the check so we accept IDs from 12 to 40 chars in
length.

Cc: Geert Uytterhoeven <geert@linux-m68k.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>