net: sgi: ioc3-eth: Fix return value check in ioc3eth_probe()

In the function devm_platform_ioremap_resource(), if get resource
failed, the return value is ERR_PTR() not NULL. Thus it must be
replaced by IS_ERR(), or else it may result in crashes if a critical
error path is encountered.

Fixes: 0ce5ebd2 ("mfd: ioc3: Add driver for SGI IOC3 chip")
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>