staging: comedi: adv_pci_dio: fix bug in 'detach'

The 'attach' function can fail between when the devpriv and
the comedi subdevices are allocated. If it does the 'detach'
will try to access unallocated memory when it goes thru the
subdevices.

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>