staging: comedi: comedi_fops: simplify comedi_is_subdevice_idle()

Don't use a conditional operator when a simple "not" will do.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>