usb: xhci: Fix build warning seen with CONFIG_PM=n

The following build warning is seen if CONFIG_PM is disabled.

drivers/usb/host/xhci-pci.c:498:13: warning:
	unused function 'xhci_pci_shutdown'

Fixes: f2c710f7 ("usb: xhci: only set D3hot for pci device")
Cc: Henry Lin <henryl@nvidia.com>
Cc: stable@vger.kernel.org	# all stable releases with f2c710f7


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20191218011911.6907-1-linux@roeck-us.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>