Define the maximum payload size for a message.

Change-Id: I3e4dd9a0fc075358c3b2f1a1cccb89a523d96c31
diff --git a/src/api.c b/src/api.c
index daa6a81..ad566e2 100644
--- a/src/api.c
+++ b/src/api.c
@@ -767,7 +767,7 @@
 
 	size = from_msg_replica.length;
 	/* Limit the size of transfer. */
-	if (size > HF_MAILBOX_SIZE - sizeof(struct spci_message)) {
+	if (size > SPCI_MSG_PAYLOAD_MAX) {
 		return SPCI_INVALID_PARAMETERS;
 	}