staging: comedi: pcmuio: add inline helpers to get the 'iobase', 'asic', and 'port'

To reduce the potential for bugs, and better document the code, introduce
some inline helper functions to consolidate the calculations needed to get
the 'iobase' for a given asic and the 'asic' and 'port' associated with a
given subdevice.

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>