Restore the ability for i2c slave devices to do IO from their probe method.

Restore the ability for i2c slave devices to do IO from their probe method.

r348164 added code to iicbus_request_bus/iicbus_release_bus to automatically
call device_busy()/device_unbusy() as part of aquiring exclusive use of the
bus (so modules can't be unloaded while the bus is exclusively owned and/or
IO is in progress).  That broke the ability to do i2c IO from a slave device
probe method, because the slave isn't attached yet, so calling device_busy()
triggers a sanity-check panic for trying to busy a non-attached device.

Now we check whether the device status is < DS_ATTACHING, and if so we busy
the iicbus rather than the slave device.  I think this leaves a small window
where a module could be unloaded while probing is in progress.  But I think
that's true of all devices, and probably should be fixed by introducing a
DS_PROBING state for devices, and handling that at various points in the
newbus code.