staging: comedi: mf6x4: rename remaining BAR1 register defines

For consistency, rename the ADSTART and DAC register defines to add
a bit of clarity.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
 drivers/staging/comedi/drivers/mf6x4.c | 16 ++++++++--------
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>