PCI: brcmstb: Fix window register offset from 4 to 8

PCI: brcmstb: Fix window register offset from 4 to 8

The outbound memory window registers were being referenced
with an incorrect stride offset.  This probably wasn't noticed
previously as there was likely only one such window employed.

Link: https://lore.kernel.org/r/20200507201544.43432-3-james.quinlan@broadcom.com
Fixes: c0452137034b ("PCI: brcmstb: Add Broadcom STB PCIe host controller driver")
Signed-off-by: Jim Quinlan <jquinlan@broadcom.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>