drivers: can: remove option for configuring CAN-FD max DLC

Remove the Kconfig option for configuring the maximum value of the CAN-FD
Data Length Code in order to ensure standard compliance.

Allowing users to configure the max DLC allows the user to save a number of
bytes per CAN frame in RAM, but it also renders the Zephyr CAN-FD
implementation non-compliant with the CAN-FD standard.

There are no guarantees that a Zephyr CAN-FD node will not receive a CAN
frame with a DLC higher than the Kconfig limit, which opens up for all
sorts of bugs when trying to handle received CAN-FD frames.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>