net: Fix return value about devm_platform_ioremap_resource()

When call function devm_platform_ioremap_resource(), we should use IS_ERR()
to check the return value and return PTR_ERR() if failed.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>