drm/amd/display: use a more appropriate return value in dp_retrieve_lttpr_cap()

Not all ASICs support LTTPR, however if they don't it doesn't mean that
we have encountered unexpected behaviour. So, use DC_NOT_SUPPORTED
instead of DC_ERROR_UNEXPECTED.
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/link/link_dp_capability.c

@@ -1462,7 +1462,7 @@ enum dc_status dp_retrieve_lttpr_cap(struct dc_link *link)
 	bool vbios_lttpr_interop = link->dc->caps.vbios_lttpr_aware;
 
 	if (!vbios_lttpr_interop || !link->dc->caps.extended_aux_timeout_support)
-		return DC_ERROR_UNEXPECTED;
+		return DC_NOT_SUPPORTED;
 
 	/* By reading LTTPR capability, RX assumes that we will enable
 	 * LTTPR extended aux timeout if LTTPR is present.