Setting an alt_speed using the ASYNC_SPD flags has been deprecated since
v2.1.69, and has been broken since v3.10 and commit 6865ff22 ("TTY:
do not warn about setting speed via SPD_*") without anyone noticing.

Drop the broken alt-speed handling altogether, and add a ratelimited
warning about using TIOCCSERIAL to change speed as being deprecated.

Note that using ASYNC_SPD_CUST is still supported.
Signed-off-by: NJohan Hovold <johan@kernel.org>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>