ieee802154: change to new flag variable

Replace EXTRA_CFLAGS with ccflags-y.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Acked-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

drivers/ieee802154/Makefile

@@ -1,3 +1,3 @@
 obj-$(CONFIG_IEEE802154_FAKEHARD) += fakehard.o
 
-EXTRA_CFLAGS += -DDEBUG -DCONFIG_FFD
+ccflags-y := -DDEBUG -DCONFIG_FFD