tty: Document defunct ASYNC_SPLIT_TERMIOS flag in uapi header

The last vestige of ASYNC_SPLIT_TERMIOS was removed by commit
'cris: Remove obsolete ASYNC_SPLIT_TERMIOS behavior'. Mark the flag
as defunct in the uapi header.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>