platforms/ibm-fsp/firenze: Refactor firenze_pci_setup_power_mgt()
platforms/ibm-fsp/firenze: Refactor firenze_pci_setup_power_mgt()

This refactors firenze_pci_setup_power_mgt() and no logicial changes
introduced:

   * Avoid nested if block;
   * Remove the local variable @buddy. The information is retrieved
     directly from struct firenze_pci_slot_info::buddy;
   * Comments to explain the fixed register offset (0x69) for slot's
     power control and shared power state between local slot and its
     buddy.

Signed-off-by: Gavin Shan <0398910e4f4970259b451f0f9d0686cc5e961b81@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

legoater/skiboot,mikey/skiboot,stewart-ibm/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,mikey/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,mikey/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,open-power/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,stewart-ibm/skiboot,legoater/skiboot