can: flexcan: Deferred on Regulator return EPROBE_DEFER

Return EPROBE_DEFER if Regulator returns EPROBE_DEFER

If the Flexcan driver is built into kernel and a regulator is used to
enable the CAN transceiver, the Flexcan driver may not use the regulator.

When initializing the Flexcan device with a regulator defined in the device
tree, but not initialized, the regulator subsystem returns EPROBE_DEFER, hence
the Flexcan init fails.

The solution for this is to return EPROBE_DEFER if regulator is not initialized
and wait until the regulator is initialized.

Signed-off-by: Andreas Werner <kernel@andy89.org>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>