can: flexcan: flexcan_probe(): make regulator xceiver optional

As the transcevier regulator is optional, this patch switches from
devm_regulator_get() to devm_regulator_get_optional(). This gets rid of "using
dummy regulator" warning message from the regulator core, if no regulator is
available.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200922144429.2613631-8-mkl@pengutronix.de