fix(intel): change SMC return arguments for INTEL_SIP_SMC_MBOX_SEND_CMD

'INTEL_SIP_SMC_MBOX_SEND_CMD' SMC runtime service will only return
mailbox status and the argument's length back to the caller

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Change-Id: I50d2ae74845794cab7bf0858e742b5a70e0ea868
diff --git a/plat/intel/soc/common/socfpga_sip_svc.c b/plat/intel/soc/common/socfpga_sip_svc.c
index 66dd6cd..a9e3ce3 100644
--- a/plat/intel/soc/common/socfpga_sip_svc.c
+++ b/plat/intel/soc/common/socfpga_sip_svc.c
@@ -528,7 +528,7 @@
 		status = intel_mbox_send_cmd(x1, (uint32_t *)x2, x3, x4,
 					     (uint32_t *)x5, x6, &mbox_status,
 					     &len_in_resp);
-		SMC_RET4(handle, status, mbox_status, x5, len_in_resp);
+		SMC_RET3(handle, status, mbox_status, len_in_resp);
 
 	default:
 		return socfpga_sip_handler(smc_fid, x1, x2, x3, x4,