drivers: net: davinci_mdio: fix return value check in davinci_mdio_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: 03f66f06 ("net: ethernet: ti: davinci_mdio: use devm_ioremap()")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>