[PATCH] constify tty flip buffer handling

Add a couple of 'const' qualifiers to the TTY flip buffer APIs, where
appropriate.

Signed-off-by: Thomas Koeller <thomas@koeller.dyndns.org>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>