amiserial: simplify rs_open

amiserial: simplify rs_open

tty->port is already set when rs_open is called given we linked it by
tty_port_link_device(). If it wasn't, the tty layer would WARN loudly.
So it's pointless to set it in rs_open. Instead, use the value in
tty->port to find out the serial_state (info).

It's a fallout of commit b19e2ca77ee4 (TTY: use tty_port_link_device)
which added tty_port_link_device here, but omitted to remove the
tty->port assignment from rs_open.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210714091314.8292-5-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>