staging: comedi: s626: fix value written by s626_set_dac()

I broke `s626_set_dac()` by changing the type of the `dacdata` parameter
from `short` to `unsigned short`.  It's actually designed to take a
signed value in the range -0x1fff to +0x2000 although values above
0x1fff get clamped to 0x1fff.  (We could change the `maxdata` value to
0x1ffe to avoid the clamping, but `maxdata` values are usually a power
of 2 minus 1.)  The bug results in all negative values passed to the
function being changed to +0x1fff by the clamp.  Change the parameter
type to `int16_t` to fix the problem.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>