amiserial: switch rs_table to a single state

amiserial deals only with a single serial, so drop the rs_table array
and NR_PORTS and define a single non-array serial_state for simplicity
instead.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210714091314.8292-10-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>