PCI: dwc: designware: Get device pointer at the start of dw_pcie_host_init()

No functional change. Get device pointer at the beginning of
dw_pcie_host_init() instead of getting it all over dw_pcie_host_init().
This is in preparation for splitting struct pcie_port into host and core
structures (once split pcie_port will not have device pointer).

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>