staging: comedi: das08: Correct AO output for das08jr-16-ao

Don't zero out bits 15..12 of the data value in `das08jr_ao_winsn()` as
that knobbles the upper three-quarters of the output range for the
'das08jr-16-ao' board.

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