staging: comedi: dt3000: remove 'phys_addr' from the private data

The 'phys_addr' variable in the private data is simply used as
a flag for the detach function to know that the pci device has
been enabled. Use the 'dev->iobase' variable instead as is more
typical for other comedi pci drivers.

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>