staging: comedi: unioxx5: fix unioxx5_detach()

During the attach of this driver, it's possible for the allocation
of the subdevice private data to fail. It's also possible that the
io region was not successfully requested.

Validate the pointer and iobase before trying to release the region.

For aesthetic reasons, rename the local variables.

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>