pds_core: Fix documentation for pds_client_register

pds_core: Fix documentation for pds_client_register

The documentation above pds_client_register states that it returns 0 on
success and negative on error. However, it actually returns a positive
client ID on success and negative on error. Fix the documentation to
state exactly that.

Signed-off-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/20230801165833.1622-1-brett.creeley@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>