can: c_can: checking IS_ERR() instead of NULL

devm_ioremap() returns NULL on error, not an ERR_PTR().

Fixes: 33cf7565 ('can: c_can_platform: Fix raminit, use devm_ioremap() instead of devm_ioremap_resource()')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: linux-stable <stable@vger.kernel.org> # >= v3.11
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>