PCI: keystone: Fix error handling when "num-viewport" DT property is not populated

Fix error handling when "num-viewport" DT property is not populated.

Fixes: 23284ad6 ("PCI: keystone: Add support for PCIe EP in AM654x Platforms")
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org # v5.2+