can: at91_can: fix compiler warning in at91_irq_err_state

This patch fixes the following compiler warning:

drivers/net/can/at91_can.c: In function 'at91_irq_err_state':
drivers/net/can/at91_can.c:779: warning: 'reg_ier' may be used uninitialized in this function
drivers/net/can/at91_can.c:779: warning: 'reg_idr' may be used uninitialized in this function

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>