checkpatch: fix perl version 5.12 and earlier incompatibility

A previous patch ("checkpatch: add --types option to report only
specific message types") uses a perl syntax introduced in perl version
5.14.

Use the backward compatible perl syntax instead.

Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>