tile PCI RC: cleanups for tilepro PCI RC

- remove unneeded <linux/bootmem.h> include in pci.c
- eliminate unused pci_controller.first_busno field
- prefer msleep to mdelay
- remove stale comment about pci_scan_bus_parented()

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>