rpmsg: don't return error when no more buffer available but wait

If no buffer is available _rpmsg_virtio_get_buffer_size returns 0.
in this case "size > avail_size" is true so we leave the function
without waiting buffer release. Patch consists in ignoring the
test if avail_size is null.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>

lib/rpmsg/rpmsg_virtio.c

@@ -308,7 +308,7 @@ static int rpmsg_virtio_send_offchannel_raw(struct rpmsg_device *rdev,
 		/* Lock the device to enable exclusive access to virtqueues */
 		metal_mutex_acquire(&rdev->lock);
 		avail_size = _rpmsg_virtio_get_buffer_size(rvdev);
-		if (size > avail_size) {
+		if (avail_size && size > avail_size) {
 			metal_mutex_release(&rdev->lock);
 			return RPMSG_ERR_BUFF_SIZE;
 		}