PCI: xilinx-cpm: Fix reference count leak on error path

Also drop the reference count of the node on error path.

Link: https://lore.kernel.org/r/20210120143745.699-1-bianpan2016@163.com
Fixes: 508f6106

 ("PCI: xilinx-cpm: Add Versal CPM Root Port driver")
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>