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

commit 61ed59ed09e6ad2b8395178ea5ad5f653bba08e3 upstream.

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.

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