checkpatch: use a more consistent function argument style

Instead of array indexing $_, use temporary variables like all the other
subroutines in the script use.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>