staging: comedi: pcmmio: remove 'asic_chan' from subdevice private data

The first 'asic_chan' that supports interrupts in the interrupt subdevice is
always initialized to '0'. Remove this unneeded information from the subdevice
private data.

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>