checkpatch: case/default checks should only check changed lines

We should only be checking changes lines for the trailing statement check
on case/default statements.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>