PCI: Cleanup setup-bus.c comments and whitespace

Cleanup comments, kernel-doc, coding style.  No functional changes
intended; comment and whitespace changes only.

Link: https://lore.kernel.org/lkml/PS2P216MB06427E290A68CDB921FB4B2980250@PS2P216MB0642.KORP216.PROD.OUTLOOK.COM


Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
[bhelgaas: tidy related things throughout the file]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>