can: at91_can: use endian agnostic IO accessors

Change __raw accesors to endian agnostic versions to allow the driver
to work properly on big endian systems.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>