staging: comedi: pcl816: all board types have 16 analog inputs

All the boards supported by this driver have 16 analog input channels.

Remove the 'n_aichan' member from the boardinfo and refactor pcl816_attach().

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>