can: mcp251xfd: mcp251xfd_start_xmit(): use mcp251xfd_get_tx_free() to check TX is is full

This patch replaces an open coded check if the TX ring is full by a check if
mcp251xfd_get_tx_free() returns 0.

Link: https://lore.kernel.org/r/20210114153448.1506901-2-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c

@@ -2436,7 +2436,7 @@ static netdev_tx_t mcp251xfd_start_xmit(struct sk_buff *skb,
 	/* Stop queue if we occupy the complete TX FIFO */
 	tx_head = mcp251xfd_get_tx_head(tx_ring);
 	tx_ring->head++;
-	if (tx_ring->head - tx_ring->tail >= tx_ring->obj_num)
+	if (mcp251xfd_get_tx_free(tx_ring) == 0)
 		netif_stop_queue(ndev);
 
 	can_put_echo_skb(skb, ndev, tx_head, 0);