can: flexcan: implement workaround for errata ERR005829

This patch implements the workaround mentioned in ERR005829:

    ERR005829: FlexCAN: FlexCAN does not transmit a message that is enabled to
    be transmitted in a specific moment during the arbitration process.

Workaround: The workaround consists of two extra steps after setting up a
message for transmission:

Step 8: Reserve the first valid mailbox as an inactive mailbox (CODE=0b1000).
If RX FIFO is disabled, this mailbox must be message buffer 0. Otherwise, the
first valid mailbox can be found using the "RX FIFO filters" table in the
FlexCAN chapter of the chip reference manual.

Step 9: Write twice INACTIVE code (0b1000) into the first valid mailbox.

Signed-off-by: David Jander <david@protonic.nl>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>