USB: fix build error with CONFIG_PM_RUNTIME disabled

commit bdd405d2 ("usb: hub: Prevent hub autosuspend if
usbcore.autosuspend is -1") causes a build error if CONFIG_PM_RUNTIME is
disabled.  Fix that by doing a simple #ifdef guard around it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Roger Quadros <rogerq@ti.com>
Cc: Michael Welling <mwelling@emacinc.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>