net: korina: Fix return value check in korina_probe()

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

Fixes: b4cd249a ("net: korina: Use devres functions")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>