can: flexcan: enable interrupts atomically at the end of flexcan_chip_start()

This patch defers the writing of the interrupts bits of the CTRL register order
to enables all interrupts atomically at the the of the flexcan_chip_start()
function.

Suggested-by: Torsten Lang <torsten.lang@uweschneider.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>