platform/x86/intel/pmt: use guard(mutex)

platform/x86/intel/pmt: use guard(mutex)

Update the mutex paths to use the new guard() mechanism.

With the removal of goto, do some minor cleanup of the current logic
path.

Reviewed-by: David E. Box <david.e.box@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://lore.kernel.org/r/20250713172943.7335-7-michael.j.ruhl@intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>