usb: Add support for indicating whether a port is removable

Userspace may want to make policy decisions based on whether or not a
given USB device is removable. Add a per-device member and support
for exposing it in sysfs. Information sources to populate it will be
added later.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>