staging: comedi: dt2811: fix a precedence bug

Bitwise | has higher precedence than ?: so we need to add some
parenthesis for this to work as intended.

Fixes: 7c957409 ('staging: comedi: dt2811: simplify A/D reference configuration')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>