staging: comedi: das08: Add number of DI channels in board structure

Add member `di_nchan` to `struct das08_board_struct` to set the number
of digital input channels explicitly.  Currently, the code determines
the number of channels to 3 or 8 depending on what function the `di`
member points to, which is not very elegant.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>