rpmsg_virtio: fix rpmsg_virtio_get_tx_payload_buffer() error

If rpmsg_virtio_notify_wait() returns RPMSG_SUCCESS, we should not
directly return NULL instead call rpmsg_virtio_get_tx_buffer to get
the tx buffer again.

Signed-off-by: Yongrong Wang <wangyongrong@xiaomi.com>
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

lib/rpmsg/rpmsg_virtio.c

@@ -394,7 +394,7 @@ static void *rpmsg_virtio_get_tx_payload_buffer(struct rpmsg_device *rdev,
 		if (status == RPMSG_EOPNOTSUPP) {
 			metal_sleep_usec(RPMSG_TICKS_PER_INTERVAL);
 			tick_count--;
-		} else if (status == RPMSG_SUCCESS) {
+		} else if (status != RPMSG_SUCCESS) {
 			break;
 		}
 	}