LU-3661 dt: Fix assertion of method in dt_capa_get()

dt_capa_get() should assert that do_capa_get method is defined
rather than do_ref_del.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: Ibeb3f3dcd0bd9d8783e87be8c6d82c4e3e5bd460
Reviewed-on: http://review.whamcloud.com/7171
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>