When make with EXTRA_CFLAGS=-W, it will report error.
  so give a check in Makefile.
Signed-off-by: NChen Gang <gang.chen@asianux.com>
Signed-off-by: NVladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Acked-by: NRob Clark <robdclark@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>