staging: comedi: cb_pcidas: refactor 'ai_bits' boardinfo

The boards supported by this driver either have 16-bit or 12-bit
analog input/output resolution. For aesthetics, replace the 'ai_bits'
member of the boardinfo with a bit-field flag 'is_16bit'. Also,
change the types of the other bit-field flags to unsigned int.

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>