Char: mxser, globals cleanup

- remove unused mxvar_diagflag
- move mxser_msr into the only user/function
- GMStatus, hmm, fix race-prone access to it. We need only one instance for
  real, not MXSER_PORTS. Move it to MOXA_GETMSTATUS ioctl.
- mxser_mon_ext, almost the same, but alloc it on heap, since it has more than
  2 kilos.
- fix indexing, `i' is not the index value, `i * MXSER_PORTS_PER_BOARD + j' is

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>