can: flexcan: flexcan_irq(): fix indention

The patch fixes the indention by replacing space by tabs, as noted by
checkpatch:

| ERROR: code indent should use tabs where possible
| #980: FILE: drivers/net/can/flexcan.c:980:

Fixes: da49a807

 ("can: flexcan: implement error passive state quirk")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>