staging: comedi: pcmuio: simplify interrupt subdevice init

Only subdevices 0 and 2 support interrupts. Simplify the attach
a bit.

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>