can: ti_hecc: fix return value check in ti_hecc_probe()

In case of error, the function devm_ioremap_resource() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: dabf54dd ("can: ti_hecc: Convert TI HECC driver to DT only driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>