hamradio: add missing sanity check to tty operation

Add missing sanity check to tty operation.

Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Eugene Teo <eteo@redhat.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>