tty: rocket: drop broken alt-speed support

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

To make things worse commit 6df3526b ("rocket: first pass at termios
reporting") in v2.6.25 started reporting back the actual baud rate used,
something which also required 38400 to again be set whenever changing a
SPD flag.

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

Note that the rocket driver has never supported using a custom divisor
(ASYNC_SPD_CUST equivalent).

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>