tty: Fix an ircomm warning and note another bug

Roel Kluin noted that line is unsigned so one test is unneccessary. Also
add a warning for another flaw I noticed while making this change.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 file changed