usb: Fix linker errors with CONFIG_PM=n

Fix these linker errors when CONFIG_PM=n:

ERROR: "xhci_bus_resume" [drivers/usb/host/xhci-hcd.ko] undefined!
ERROR: "xhci_bus_suspend" [drivers/usb/host/xhci-hcd.ko] undefined!

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>