staging: comedi: comedi_fops: introduce comedi_is_subdevice_idle()

Introduce, and use, a helper to check the subdevice runflags to see if
it is not in error and not running.

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