staging: usbip: trigger driver probing after unbinding from usbip-host

A sysfs attribute is used to announce kernel space that a
new driver probing session should be triggered for the just
unbinded device.

In order to have the address of struct device associated to this
USB device, a new member has been added to struct bus_id_priv.

Signed-off-by: Valentina Manea <valentina.manea.m@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>