staging: comedi: das08: rename and rewrite DAS08_IRQ

The `DAS08_IRQ` macro contains a mask for the "IRQ" status bit.  This is
set to 1 when a rising edge is detected on the external interrupt input
pin of the external connector (which may be jumpered to a pacer output).
It is cleared by setting the "INTE" control bit to 0.  It is not used on
"JR" boards.  Rename the macro to `DAS08_STATUS_IRQ` and add a comment.
Also use 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>