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

drivers/net/can/at91_can.c

@@ -347,8 +347,8 @@ static void at91_setup_mailboxes(struct net_device *dev)
 	u32 reg_mid;
 
 	/* Due to a chip bug (errata 50.2.6.3 & 50.3.5.3) the first
-	 * mailbox is disabled. The next 11 mailboxes are used as a
-	 * reception FIFO. The last mailbox is configured with
+	 * mailbox is disabled. The next mailboxes are used as a
+	 * reception FIFO. The last of the RX mailboxes is configured with
 	 * overwrite option. The overwrite flag indicates a FIFO
 	 * overflow.
 	 */
@@ -369,7 +369,7 @@ static void at91_setup_mailboxes(struct net_device *dev)
 		at91_write(priv, AT91_MID(i), AT91_MID_MIDE);
 	}
 
-	/* The last 4 mailboxes are used for transmitting. */
+	/* The last mailboxes are used for transmitting. */
 	for (i = get_mb_tx_first(priv); i <= get_mb_tx_last(priv); i++)
 		set_mb_mode_prio(priv, i, AT91_MB_MODE_TX, 0);