staging: comedi: das08: rename and rewrite DAS08_IP

The `DAS08_IP()` macro takes a value read from the status register and
returns the state of the three digital input channels (except on "JR"
boards).  Rename it to `DAS08_STATUS_DI()` and add a comment.  Also
re-arrange the expression used to extract the state of the digital
inputs for consistency with other register macros.

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>