PCI: xgene: Fix the xgene_msi_probe() return code

If devm_ioremap_resource() detects an error condition in the return
value through IS_ERR(), the return value should be retrieved through
PTR_ERR() instead of hardcoding it.

Fix the xgene_msi_probe() error return code.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
[lorenzo.pieralisi@arm.com: rewrote commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>