IB/cm: Correctly free cm_device structure

commit 110cf374

 ("infiniband: make cm_device use a struct device and
not a kobject.") introduced a memory leak, since it deleted
cm_release_dev_obj(), which was where cm_dev was freed.  Fix this by
freeing the leaked structure after calling device_unregister().

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>