staging: comedi: das08: Remove function pointers from board structure

Remove the `ai`, `ao`, `di` and `do` pointers from `struct
das08_board_struct`.  These were initialized in `das08_boards[]` and
`das08_cs_boards[]` to point to comedi instruction handler functions for
the subdevice, but there are only two sets of functions depending on
whether the `is_jr` member is true or false, and some of the functions
will be NULL if the corresponding `ai_nbits`, `ao_nbits`, `di_nchan` and
`do_nchan` members are zero.  Determine which handler functions to use
in das08_common_attach().

One element of `das08_boards[]` had `ao` set to NULL and `ao_nbits` set
to 12.  Set `ao_nbits` to 0 in this case to let das08_common_attach()
know this board has no AO subdevice.

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