mxser: Remove use of dead TTY_FLIPBUF_SIZE definition

We simply define it to the same value.  Nowdays the TTY flip value is
irrelevant but the value it used is as good as any so why risk breaking it

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>