staging: comedi: das16m1: tidy up analog input data register defines

Convert the inline helper munge_sample() into a macro and rename the
defines for the analog input data register/bits. Use the register define
when accessing this register instead of just dev->iobase.

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>