PCI: cadence: Simplify code to get register base address for configuring BAR

No functional change. Simplify code to get register base address for
configuring PCI BAR.

Link: https://lore.kernel.org/r/20210819123343.1951-6-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>