mxser: drop ISA support

While mxser PCI cards are still around and produced (Moxa provided me
with two recently), ISA cards are obsolete for a long time. I haven't
seen anyone using the cards and the ISA code paths are barely tested.

Hence, remove ISA support from mxser driver so that we can clean the
driver up.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-2-jslaby@suse.cz


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