staging: comedi: das08: rename and rewrite DAS08_EOC

The `DAS08_EOC` macro contains a mask for the "end of A/D conversion"
bit in the status register.  The logic is reverse sense in that the bit
is set to 1 while the conversion is in progress and set to 0 when the
conversion is complete.  Rename the macro to `DAS08_STATUS_AI_BUSY` and
add a comment.  Also make use of the `BIT()` macro to define the value.

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