usb: xhci: fix build warning - missing prototype

Fix build warning when building drivers/usb/host/xhci-mem.o with W=1 due
to missing prototype for xhci_free_virt_devices_depth_first.

This function is only used in xhci-mem.c so just make it static.

Signed-off-by: Jean-Philippe Menil <jpmenil@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>