platform/x86/intel/pmc/mtl: Use return value from pmc_core_ssram_init()

platform/x86/intel/pmc/mtl: Use return value from pmc_core_ssram_init()

Instead of checking for a NULL regbase, use the return value from
pmc_core_ssram_init() to check if PMC discovery was successful. If not, use
the legacy enumeration method (which only works for the primary PMC).

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20231129222132.2331261-15-david.e.box@linux.intel.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>