can: at91_can: rename struct at91_priv::{tx_next,tx_echo} to {tx_head,tx_tail}

To increase code readability, use the same naming of the counters for
the TX FIFO as in the other drivers implementing the same algorithm.

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-12-9987d53600e0@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>