mxser: remove MOXA_GETMSTATUS ioctl

1) MOXA_GETMSTATUS is broken at least since commit ecbbfd44 (TTY:
   move tty buffers to tty_port) in 2012.
   Even though it's mentioned in the mon_pa moxa utility, obviously
   noone ever tried that in that timeframe.
2) The ioctl definition was never exposed to userspace, so noone
   should actually be reliably using this.

Provided the above, remove this mxser-special ioctl.

And remove the whole concept of mxser_ioctl_special as this was the last
ioctl in there.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-29-jslaby@suse.cz


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