platform/x86: intel_pmc_core: Respect error code of kstrtou32_from_user()

kstrtou32_from_user() may return different error codes on certain
circumstances. Respect all possible values.

While here, move it out of the lock: there is no data access that lock guards.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>