Char: mxser, remove special baudrate processing

Let the special baudrate processing on the tty layer.  Also remove
set/get_special_rate ioctls introduced in commit
f64c84a1, since it is no longer needed.

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