Use SPCI_RX_RELEASE rather than hf_mailbox_clear.

Bug: 132421502
Change-Id: Ib80db1e310c563ee9ae7a607db986190fd45ca2d
diff --git a/main.c b/main.c
index f51c463..fb32660 100644
--- a/main.c
+++ b/main.c
@@ -327,7 +327,7 @@
 exit:
 	sock_put(&hsock->sk);
 
-	if (hf_mailbox_clear() == 1)
+	if (spci_rx_release().func == SPCI_RX_RELEASE_32)
 		hf_notify_waiters(HF_PRIMARY_VM_ID);
 }