serial: core: Controller id cannot be negative

The controller id cannot be negative. Let's fix the ctrl_id in preparation
for adding port_id to fix the device name.

Fixes: 84a9582f ("serial: core: Start managing serial controllers to enable runtime PM")
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230725054216.45696-2-tony@atomide.com


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