can: flexcan: avoid calling usleep_range from interrupt context

Apparently can_restart() runs from a (timer-) interrupt and can call
flexcan_chip_[en|dis]able(), so avoid using usleep_range()

Signed-off-by: David Jander <david@protonic.nl>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>