can: at91: remove at91_can_data

struct at91_can_data was used to pass a callback to the driver, allowing it
to switch the transceiver on and off. As all at91 boards are now using DT,
this is not used anymore, remove that structure.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>