serial: 8250_exar: Don't return positive values as error codes

serial: 8250_exar: Don't return positive values as error codes

PCI core may return positive codes for the specific errors.
There is a special API to convert them to negative stanrard ones.
Use that API to avoid returning positive values as error codes.

Fixes: f7ce07062988 ("serial: exar: add CTI specific setup code")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Parker Newman <pnewman@connecttech.com>
Link: https://lore.kernel.org/r/20240503171917.2921250-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>