staging: comedi: aio_aio12_8: refactor boardininfo

This driver supports three board types with these differences:
  104-AIO12-8 - eight 12-bit analog in, four 12-bit analog out
  104-AI12-8  - eight 12-bit analog in
  104-AO12-4  - four 12-bit analog out

Convert the boardinfo 'ai_nchan' and 'ao_nchan' into bit-field
flags 'has_ai' and 'has_ao' so save a bit of space.

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>