net: eth: xgene: devm_ioremap() returns NULL on error

devm_ioremap() returns NULL on failure, it doesn't return an ERR_PTR.

Fixes: de7b5b3d

 ('net: eth: xgene: change APM X-Gene SoC platform ethernet to support ACPI')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>