PCI: pciehp: Assign ctrl->slot_ctrl before writing it to hardware

Shameerali reported that running v4.20-rc1 as QEMU guest, the PCIe hotplug
port times out during boot:

  pciehp 0000:00:01.0:pcie004: Timeout on hotplug command 0x03f1 (issued 1016 msec ago)
  pciehp 0000:00:01.0:pcie004: Timeout on hotplug command 0x03f1 (issued 1024 msec ago)
  pciehp 0000:00:01.0:pcie004: Failed to check link status
  pciehp 0000:00:01.0:pcie004: Timeout on hotplug command 0x02f1 (issued 2520 msec ago)

The issue was bisected down to commit 720d6a67 ("PCI: pciehp: Do not
handle events if interrupts are masked") and was further analyzed by the
reporter to be caused by the fact that pciehp first updates the hardware
and only then cache the ctrl->slot_ctrl in pcie_do_write_cmd().  If the
interrupt happens before we cache the value, pciehp_isr() reads value 0 and
decides that the interrupt was not meant for it causing the above timeout
to trigger.

Fix by moving ctrl->slot_ctrl assignment to happen before it is written to
the hardware.

Fixes: 720d6a67

 ("PCI: pciehp: Do not handle events if interrupts are masked")
Link: https://lore.kernel.org/linux-pci/5FC3163CFD30C246ABAA99954A238FA8387DD344@FRAEML521-MBX.china.huawei.com
Reported-by: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>