phy: ti: j721e-wiz: Fix return value check in wiz_probe()

In case of error, the function devm_ioremap() returns NULL pointer not
ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Fixes: 091876cc ("phy: ti: j721e-wiz: Add support for WIZ module present in TI J721E SoC")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>