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: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-71-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -12389,7 +12389,8 @@ F:	drivers/media/pci/meye/
 F:	include/uapi/linux/meye.h
 
 MOXA SMARTIO/INDUSTIO/INTELLIO SERIAL CARD
-S:	Orphan
+M:	Jiri Slaby <jirislaby@kernel.org>
+S:	Maintained
 F:	Documentation/driver-api/serial/moxa-smartio.rst
 F:	drivers/tty/mxser.*