staging: comedi: das16: all supported board have analog inputs

All the boards supported by this driver have analog inputs and all
of then use the same function to handle the (*insn_read) for the
subdevice.

Remove the 'ai' member from the boardinfo and always initalize the
analog input subdevice during the board attach.

Tidy up the subdevice init a bit.

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>