MAINTAINERS: add me back as mxser maintainer

I was given a few cards from MOXA (thanks a lot). Provided I can now
test changes, I cleaned up the driver a bit and can continue maintaining
it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-71-jslaby@suse.cz


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