staging: comedi: addi_apci_1564: use correct I/O base for APCI1564_DI_INT_STATUS_REG

The APCI1564_DI_INT_STATUS_REG is located in the PCI BAR 0 I/O space. That
base address is stored in devpriv->amcc_iobase. Use that to correctly read
the register.

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>