PCI: pciehp: Reshuffle controller struct for clarity
The members in pciehp's controller struct are arranged in a seemingly
arbitrary order and have grown to an amount that I no longer consider
easily graspable by contributors.
Sort the members into 5 rubrics:
* Slot Capabilities register and quirks
* Slot Control register access
* Slot Status register event handling
* state machine
* hotplug core interface
Obviously, this is just my personal bikeshed color and if anyone has a
better idea, please come forward. Any ordering will do as long as the
information is presented in a manageable manner.