PCI: j721e: Initialize pcie->cdns_pcie before using it

PCI: j721e: Initialize pcie->cdns_pcie before using it

Christian reported a NULL pointer dereference in j721e_pcie_probe() caused
by 19e863828acf ("PCI: j721e: Drop redundant struct device *"), which
removed struct j721e_pcie.dev since there's another copy in struct
cdns_pcie.dev reachable via j721e_pcie->cdns_pcie->dev.

The problem is that j721e_pcie->cdns_pcie was dereferenced before being
initialized:

  j721e_pcie_probe
    pcie = devm_kzalloc()             # struct j721e_pcie
    j721e_pcie_ctrl_init(pcie)
      dev = pcie->cdns_pcie->dev      <-- dereference cdns_pcie
    switch (mode) {
    case PCI_MODE_RC:
      cdns_pcie = ...                 # alloc as part of pci_host_bridge
      pcie->cdns_pcie = cdns_pcie     <-- initialize pcie->cdns_pcie

Move the cdns_pcie initialization earlier so it is done before it is used.
This also simplifies the error exits.

Fixes: 19e863828acf ("PCI: j721e: Drop redundant struct device *")
Link: https://lore.kernel.org/r/20220127222951.GA144828@bhelgaas
Link: https://lore.kernel.org/r/20220124122132.435743-1-christian.gmeiner@gmail.com
Reported-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Tested-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>