staging: comedi: addi_apci_1500: handle shared interrupt

The interrupt used by this driver is shared. If the board did not cause
the interrupt the driver should return IRQ_NONE so that another driver
can handle it. Fix the interrupt handler so this happens.

Tidy up the interrupt handler a bit.

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>