firmware: scmi: mailbox: use smt polling method

Since u-boot does not support interrupt, use smt polling method to
wait for req completion

Change-Id: Ic132a1918659bef8e182d3e20ba9027a3d3a49ad
Signed-off-by: Michel JAOUEN <michel.jaouen@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/u-boot/+/446889
Domain-Review: Patrick DELAUNAY <patrick.delaunay@foss.st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>
Reviewed-by: Patrice CHOTARD <patrice.chotard@foss.st.com>
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>

Author: jamike

drivers/firmware/scmi/mailbox_agent.c

@@ -57,15 +57,13 @@ static int scmi_mbox_process_msg(struct udevice *dev,
 		goto out;
 	}
 
+	ret = scmi_wait_resp_from_smt(dev, &chan->smt, msg, chan->timeout_us);
 	/* Receive the response */
-	ret = mbox_recv(&chan->mbox, chan->smt.buf, chan->timeout_us);
-	if (ret) {
+	if (ret == -ETIMEDOUT) {
 		dev_err(dev, "Response failed: %d, abort\n", ret);
 		goto out;
 	}
 
-	ret = scmi_read_resp_from_smt(dev, &chan->smt, msg);
-
 out:
 	scmi_clear_smt_channel(&chan->smt);