platform/x86:intel/vsec: Change return type of intel_vsec_register

platform/x86:intel/vsec: Change return type of intel_vsec_register

Change return type of intel_vsec_register() to int. The current
implementation does not indicate if the register fail or not.
Change to return error code if it fails or if INTEL_VSEC config
is not set. This is a preparation step to introduce a new SSRAM
Telemetry driver that will be using this API.

Signed-off-by: Xi Pardee <xi.pardee@linux.intel.com>
Link: https://lore.kernel.org/r/20250425195237.493129-2-xi.pardee@linux.intel.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>