staging: comedi: dmm32at: rename DMM32AT_AI{LOW,HIGH}

For aesthetics, rename these defines used for the A/D Low/High Channel
registers.

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>