staging: comedi: pcmuio: simplify 'n_subdevs' and 's->n_chan'

The number of subdevices needed by the driver is based on the
'num_asics' reported by the boardinfo and each subdevice always
has 24 channels. 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>