platform/x86: intel_pmc_ipc: Make intel_pmc_ipc_raw_cmd() static

This function is not called outside of intel_pmc_ipc.c so we can make it
static instead.

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>