PCI: dwc: Remove hardcoded PCI_CAP_ID_EXP offset

PCI: dwc: Remove hardcoded PCI_CAP_ID_EXP offset

While the Designware controller appears to hard code the PCI_CAP_ID_EXP
capability register at 0x70, there's no need to hard code this in the
driver as it is discoverable.

Link: https://lore.kernel.org/r/20200821035420.380495-31-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Pratyush Anand <pratyush.anand@gmail.com>
Cc: linux-omap@vger.kernel.org