staging: comedi: das16: fix das16_detach()

The function das16_reset() needs a valid dev->iobase. Since the iobase
is requested after the devpriv has been allocated in the attach, move
the das16_reset() call into the if (devpriv) and add a if (dev->iobase).

Also, move the release of the extra iobase into the if (devpriv) to
prevent an invalid memory access.

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