checkpatch: add --strict test for switch/default missing break

switch default case is sometimes written as "default:;".  This can cause
new cases added below the default to be defective.

Suggest adding a break; after empty default cases to avoid fallthrough
defects.

Fixed indentation in the other semicolon test above it.

Suggested-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>