staging: comedi: ni_mio_common: use dev->read_subdev

The (*attach) in ni_E_init() sets the dev->read_subdev to the analog
input subdevice. Use that instead of accessing the dev->subdevices
array directly.

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