platform/x86: intel_pmc_ipc: Use octal permissions in sysfs attributes

This is the current preferred way so replace the S_IWUSR with the
corresponding octal value. While there move the attributes to follow
directly their store functions.

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>