can: at91_can: at91_setup_mailboxes(): update comments

Since 6388b396 ("can: at91_can: add support for the AT91SAM9X5
SOCs") the number of mailboxes used for RX and TX is no longer
constant, but depends on the IP core used.

Remove the fixed number of mailboxes from the comment.

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-11-9987d53600e0@pengutronix.de


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