staging: comedi: quatech_daqp_cs: remove 'interrupt_mode'

The interrupt handler is now only used for the ai async command.
Remove the unnecessary 'interrupt_mode' from the private data and
tidy up the interrupt handler.

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>