driver model: add bus_rescan_devices()

driver model: add bus_rescan_devices()

From Ben Collins

Used as manual trigger for device/driver binding, so drivers can claim
devices that are present but failed to be bound. In case of ieee1394, this
can happen if two computers are attached to a device - the second one won't
be able to claim it until the first detaches.

Changed device_attach() to return 1 when a driver was bound to a device.
Makes it so bus_rescan_devices() can return the number of devices attached.
Tested this with ieee1394 over a bus reset and it worked as expected.