PCI: xgene: Fix cfg resource mapping

PCI: xgene: Fix cfg resource mapping

In commit 69c2bd0dbc09 a change was made to use
devm_platform_ioremap_resource_byname() to simplify code and remove
the res variable; this was wrong since the res variable is still needed
and as an outcome the port->cfg_addr gets an erroneous address.

Revert the change going back to original behaviour.

Link: https://lore.kernel.org/r/20210328144118.305074-1-zhengdejin5@gmail.com
Fixes: 69c2bd0dbc094 ("PCI: controller: Convert to devm_platform_ioremap_resource_byname()")
Reported-by: dann.frazier@canonical.com
Tested-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org    # v5.9+