staging: comedi: make 'dev->attached' a bool bit-field

Change the `attached` member of `struct comedi_device` to a 1-bit
bit-field of type `bool`.  Change assigned values to `true` and `false`
and replace or remove comparison operations with simple boolean tests.

We'll put some extra bit-fields in the gap later to save space.

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