staging: comedi: das08: Correct AI encoding for das08jr-16-ao

The element of `das08_boards[]` for the 'das08jr-16-ao' board has the
`ai_encoding` member set to `das08_encode12`.  It should be set to
`das08_encode16` same as the 'das08jr/16' board.  After all, this board
has 16-bit AI resolution.

The description of the A/D LSB register at offset 0 seems incorrect in
the user manual "cio-das08jr-16-ao.pdf" as it implies that the AI
resolution is only 12 bits.  The diagrams of the A/D LSB and MSB
registers show 15 data bits and a sign bit, which matches what the
software expects for the `das08_encode16` AI encoding method.

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