bcm63xx_enet: fix return value check in bcm_enet_shared_probe()

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

Introduce by commit 0ae99b5fede6f3a8d252d50bb4aba29544295219
(bcm63xx_enet: split DMA channel register accesses)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>