Essentially this patch moves the TX mailbox to position 63, regardless
of timestamp based offloading or RX FIFO. So mainly the iflag register
usage regarding TX has changed. The rest is consolidating RX FIFO and
timestamp offloading as they now use both the same TX mailbox.

The reason is a very annoying behavior regarding sending RTR frames when
_not_ using RX FIFO:

If a TX mailbox sent a RTR frame it becomes a RX mailbox. For that
reason flexcan_irq disables the TX mailbox again. But if during the time
the RTR was sent and the TX mailbox is disabled a new CAN frames is
received, it is lost without notice. The reason is that so-called
"Move-in" process starts from the lowest mailbox which happen to be a TX
mailbox set to EMPTY.

Steps to reproduce (I used an imx7d):
1. generate regular bursts of messages
2. send a RTR from flexcan with higher priority than burst messages every
   1ms, e.g. cangen -I 0x100 -L 0 -g 1 -R can0
3. notice a lost message without notification after some seconds

When running an iperf in parallel this problem is occurring even more
frequently. Using filters is not possible as at least one single CAN-ID
is allowed. Handling the TX MB during RX is also not possible as there
is no race-free disable of RX MB.

There is still a slight window when the described problem can occur. But
for that all RX MB must be in use which is essentially next to an
overrun. Still there will be no indication if it ever occurs.
Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>