can: mcp251x: Fix regulators operation without CONFIG_REGULATOR

If CONFIG_REGULATOR is not set, devm_regulator_get() returns NULL,
so use IS_ERR_OR_NULL() macro for checks.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>