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