staging: comedi: addi_apci_3501: remove "magic" numbers in apci3501_interrupt()

Use register bit defines from addi_tcw.h to remove the "magic" numbers.

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>