staging: comedi: unionxx5: fix memory leak during subdevice init

In the subdevice init during the attach of this driver, private data
is allocated for each subdevice. The pointer to this data is then
saved in the subdevice 's->private' so it can be free'ed during the
detach.

In __unioxx5_subdev_init() an error path exists that can happen before
the allocated pointer is saved in s->private. Make sure the allocated
memory is free'ed before returning the error.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>