USB: serial: ftdi_sio: remove broken alt-speed handling

Remove the broken alt_speed code, and warn when trying to set the line
speed using TIOCSSERIAL and SPD flags.

The use of SPD flags to set the line speed has been deprecated since
v2.1.69 and support for alt_speed (e.g. "warp") has even been removed
from TTY core in v3.10 by commit 6865ff22 ("TTY: do not warn about
setting speed via SPD_*"), effectively breaking all driver
implementations of this except for serial core.

Also remove the verbose and outdated comment on how to set baud rates.

Note that setting a custom divisor will continue to work with the
caveat that 38400 must again be selected every time the divisor is
changed since v2.6.24 and commit 669a6db1 ("USB: ftd_sio: cleanups
and updates for new termios work") which started reporting back the
actual baud rate used.

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>