mxser: remove MOXA_HighSpeedOn ioctl

1) The semantics are defined exactly nowhere.
2) The ioctl definition was never exposed to userspace, so noone should
   actually be reliably using this.
3) It's not even mentioned in any of the mxser utilities.

Provided the above, remove this mxser-special ioctl.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-31-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>