net: axienet: Fix devm_ioremap_resource return value check

Value returned by devm_ioremap_resource() was checked for non-NULL but
devm_ioremap_resource() returns IOMEM_ERR_PTR, not NULL. In case of
error this could lead to dereference of ERR_PTR.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Cc: <stable@vger.kernel.org>
Fixes: 46aa27df

 ("net: axienet: Use devm_* calls")
Reviewed-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>