staging: comedi: addi_apci_1516: remove devpriv->iobase usage

The iobase address stored in devpriv->iobase is also stored in dev->iobase.
Use that instead.

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