can: mcp251xfd: mcp251xfd_unregister(): simplify runtime PM handling

can: mcp251xfd: mcp251xfd_unregister(): simplify runtime PM handling

The mcp251xfd driver supports runtime PM enabled kernels, but also
works on !CONFIG_PM configurations.

This patch simplifies the runtime PM handling in the
mcp251xfd_unregister(). In the CONFIG_PM case, runtime PM has been
enabled in the mcp251xfd_probe() function, so we can disable it here.
For !CONFIG_PM builds call mcp251xfd_clks_and_vdd_disable() directly.

Link: https://lore.kernel.org/all/20220207131047.282110-4-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>