serial: 8250: Fix oops for port->pm on uart_change_pm()

Unloading a hardware specific 8250 driver can produce error "Unable to
handle kernel paging request at virtual address" about ten seconds after
unloading the driver. This happens on uart_hangup() calling
uart_change_pm().

Turns out commit 04e8279 ("serial: 8250: Reinit port->pm on port
specific driver unbind") was only a partial fix. If the hardware specific
driver has initialized port->pm function, we need to clear port->pm too.
Just reinitializing port->ops does not do this. Otherwise serial8250_pm()
will call port->pm() instead of serial8250_do_pm().

Fixes: 04e8279 ("serial: 8250: Reinit port->pm on port specific driver unbind")
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230804131553.52927-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -3278,6 +3278,7 @@ void serial8250_init_port(struct uart_8250_port *up)
 
 	spin_lock_init(&port->lock);
 	port->ctrl_id = 0;
+	port->pm = NULL;
 	port->ops = &serial8250_pops;
 	port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_8250_CONSOLE);