PCI: mediatek: Fix unchecked return value

Check return value of devm_pci_remap_iospace().

Addresses-Coverity-ID: 1471965 ("Unchecked return value")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Honghui Zhang <honghui.zhang@mediatek.com>