tty_ldisc: remove unnecessary negative return check for wait_event_timeout

wait_event_timeout always return value >= 0
remove the unnecessary ret < 0 check

Signed-off-by: Dave Young <dyoung@redhat.com>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>