staging: comedi: amplc_dio200: set dev->iobase before common attach

Don't pass the I/O base address to `dio200_common_attach()`.  The only
thing it does with it is set `dev->iobase` to the passed in value.  Do
that before calling `dio200_common_attach()` in order to simplify
upcoming support for different register access methods.

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