staging: comedi: bug-fix NULL pointer dereference on failed attach

Commit dcd7b8bd ("staging: comedi: put
module _after_ detach" by myself) reversed a couple of calls in
`comedi_device_attach()` when recovering from an error returned by the
low-level driver's 'attach' handler.  Unfortunately, that introduced a
NULL pointer dereference bug as `dev->driver` is NULL after the call to
`comedi_device_detach()`.   We still have a pointer to the low-level
comedi driver structure in the `driv` variable, so use that instead.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org> # 3.10+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>