can: tcan4x5x: add max_raw_{read,write} of 256

The tcan4x5x chip support bulk read/write, but as the length field is only 8
bits wide, the maximum is 256. A length of 0 is treated as 256.

Reviewed-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20201215231746.1132907-14-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>