platform/x86: intel_pmc_ipc: Propagate error from kstrtoul()

kstrtoul() already returns negative error if the input was not valid so
return it directly.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>