staging: comedi: daqboard2000: rename reference DACs register macros

Rename the macros that define values for the reference DACs register to
avoid CamelCase, and to make it clearer which register they are
associated with.  Add a macro `DB2K_REG_DACS_SET` for the value `0x0080`
that triggers setting one of the references.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>