staging: comedi: das08: Remove pci_iobase from private data

The `pci_iobase` I/O port registers from PCI BAR 1 are never accessed,
so don't bother storing this base address in `struct
das08_private_struct`.

The `pci_iobase` member was checked for non-zero in `das08_detach()` but
we can check `dev->iobase` is non-zero instead.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>