staging: comedi: ni_mio_common: only reset the ai/ao subdevices if they exist

So of the boards supported by this driver do not have analog inputs and
some don't have analog outputs. Move the calls that reset these subdevices
during the (*attach) by ni_E_init() so they only happen if the subdevices
are present.

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>