serial: core: Revert port_id use

Guenter reports boot issues with duplicate sysfs entries for multiport
drivers. Let's go back to using port->line for now to fix the regression.

With this change, the serial core port device names are not correct for the
hardware specific 8250 single port drivers, but that's a cosmetic issue for
now.

Fixes: d962de6a

 ("serial: core: Fix serial core port id to not use port->line")
Reported-by: Guenter Roeck <groeck7@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230806062052.47737-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>