staging: comedi: pcmuio: tidy up pcmuio_handle_asic_interrupt()

Unfortunatly, since there could be two asics, we can't use dev->read_subdev
to get the subdevice. But, the comedi_subdevice associated with the 'asic'
can easily be calculated. This allows removing the for () loop that searched
for the correct subdevice.

Tidy up the function.

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>