mxser: remove MOXA_CHKPORTENABLE ioctl

1) MOXA_CHKPORTENABLE is broken at least since commit ecbbfd44 (TTY:
   move tty buffers to tty_port) in 2012.
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/20210618061516.662-27-jslaby@suse.cz


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