staging: comedi: dmm32at: rename DMM32AT_AISTAT

For aesthetics, rename this define used for the A/D Status register. Also,
rename the defines for the bits of the register.

The probe code in dmm32at_reset() checks this register to make sure the
S/D1 and S/D0 bits are set. This check actually verifies that the board is
configured (with jumpers on the board) with all the Analog Input channels
operating in single-ended mode. Add a comment about this.

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>