staging: comedi: ni_labpc: allow board to attach without interrupt

If the interrupt is not available this driver will still work
with command support disabled for the analog input subdevice.

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>