can: at91_can: BR register: convert to FIELD_PREP()

Use FIELD_PREP() to access the individual fields of the BR register.

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-4-9987d53600e0@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>