staging: comedi: pcmuio: remove 'num_asic_chans'

This member of the subdevice private data is always initialize to
s->n_chan if the subdevice supports interrupts and is only used
in functions that can be called by the interrupt subdevice.

Simplyfy the driver a bit  by removing it and just using s->n_chan.

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>