USB: struct usb_device: change flag to bitflag

This patch (as816) changes an existing flag in the usb_device
structure to a bitflag, preparing the way for more bitflags to come
in the future.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>