staging: comedi: dmm32at: rename DMM32AT_AIRBACK

For aesthetics, rename this define used for the Analog I/O Readback register.
Define the bits of the register.

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>