openamp: rpmsg_send() shouldn't check buffer size when get buffer failed

cause _rpmsg_virtio_get_buffer_size() always return size > 0,
when role is RPMSG_MASTER.

Signed-off-by: Guiding Li <liguiding1@xiaomi.com>

lib/rpmsg/rpmsg_virtio.c

@@ -322,6 +322,7 @@ static void *rpmsg_virtio_get_tx_payload_buffer(struct rpmsg_device *rdev,
 		metal_mutex_release(&rdev->lock);
 		if (rp_hdr || !tick_count)
 			break;
+
 		metal_sleep_usec(RPMSG_TICKS_PER_INTERVAL);
 		tick_count--;
 	}