staging: comedi: pcmuio: remove 'asic' member from subdevice private data

The 'asic' associated with a subdevice can be easily calculated. The
functions that use this member in the subdevice private data can only
be called by the subdevices that support interrupts. Just calculate
the 'asic' when needed and remove the member variable and sanity checks.

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>