staging: comedi: addi_apci_1564: move apci1564_interrupt() into addi_apci_1564.c

On moving the function into the driver proper, also check the device is
asserting the shared interrupt line.

This patch also fixes the interrupt handling for the digital input
change-of-state interrupts.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>