staging: comedi: dmm32at: rename DMM32AT_DACMSB_CHAN

For aesthetics, rename this define used to set the DACH[10] bits in
the DAC MSB register to select the D/A channel.

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