staging: comedi: avoid a put_device(), get_device() sequence

In `comedi_set_hw_dev()`, if there is no change to `dev->hw_dev` (and it
is not `NULL`), don't bother putting and getting the device.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>