firmware: arm_scmi: Fix return error code in smc_send_message

SMCCC can return NOT_SUPPORTED(-1). Map it to appropriate Linux error
codes namely -EOPNOTSUPP.

Link: https://lore.kernel.org/r/20200417103232.6896-1-sudeep.holla@arm.comReported-and-Tested-by: : Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/smc.c

@@ -114,7 +114,10 @@ static int smc_send_message(struct scmi_chan_info *cinfo,
 
 	mutex_unlock(&scmi_info->shmem_lock);
 
-	return res.a0;
+	/* Only SMCCC_RET_NOT_SUPPORTED is valid error code */
+	if (res.a0)
+		return -EOPNOTSUPP;
+	return 0;
 }
 
 static void smc_fetch_response(struct scmi_chan_info *cinfo,