platform/x86: serial-multi-instantiate: Use while (i--) pattern to clean up

Use more natural while (i--) patter to clean up allocated resources.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20220709211653.18938-4-andriy.shevchenko@linux.intel.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>