staging: comedi: mf6x4: remove unnecessary defines

The MF6X4_DAC_R macro defines the offsets for all the DAC registers.
Remove the unnecessary defines for each register.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
 drivers/staging/comedi/drivers/mf6x4.c | 22 +++++++++++-----------
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>