staging: comedi: das6402: read analog input samples in interrupt handler

Currently the interrupt handler just clears the interrupt.

Add the code necessary to read the analog input samples when running
an async command.

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>