PCI: host-generic: Set driver_data before calling gen_pci_init()

On MicroChip MPFS Icicle:

  microchip-pcie 2000000000.pcie: host bridge /soc/pcie@2000000000 ranges:
  microchip-pcie 2000000000.pcie: Parsing ranges property...
  microchip-pcie 2000000000.pcie:      MEM 0x2008000000..0x2087ffffff -> 0x0008000000
  Unable to handle kernel NULL pointer dereference at virtual address 0000000000000368
  Current swapper/0 pgtable: 4K pagesize, 39-bit VAs, pgdp=0x00000000814f1000
  [0000000000000368] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
  Oops [#1]
  Modules linked in:
  CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.15.0-rc1-icicle-00003-gafc0a570bb61 #232 NONE
  Hardware name: Microchip PolarFire-SoC Icicle Kit (DT)
  [...]
  [<ffffffff803fb8a4>] plda_pcie_setup_iomems+0xe/0x78
  [<ffffffff803fc246>] mc_platform_init+0x80/0x1d2
  [<ffffffff803f9c88>] pci_ecam_create+0x104/0x1e2
  [<ffffffff8000adbe>] pci_host_common_init+0x120/0x228
  [<ffffffff8000af42>] pci_host_common_probe+0x7c/0x8a

The initialization of driver_data was moved after the call to
gen_pci_init(), while the pci_ecam_ops.init() callback
mc_platform_init() expects it has already been initialized.

Fix this by moving the initialization of driver_data up.

Fixes: afc0a570 ("PCI: host-generic: Extract an ECAM bridge creation helper from pci_host_common_probe()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/774290708a6f0f683711914fda110742c18a7fb2.1750787223.git.geert+renesas@glider.be
Link: https://patch.msgid.link/20250625111806.4153773-2-maz@kernel.org