can: at91_can: at91_irq_tx(): remove one level of indention

Improve code readability by removing one level of indention.

If a mailbox is not ready, continue the loop early.

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-3-9987d53600e0@pengutronix.de


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