staging: comedi: mf6x4: rename the digital input register defines

For aesthetics, rename these defines to clarify them a bit.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
 drivers/staging/comedi/drivers/mf6x4.c | 8 ++++----
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>