net: can: use kbuild magic to inherit debug settings

No need to manually copy debug settings into subdir Makefiles. kbuild
has a mechanism for inheriting, so let's use it.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/cc770/Makefile

@@ -5,5 +5,3 @@
 obj-$(CONFIG_CAN_CC770) += cc770.o
 obj-$(CONFIG_CAN_CC770_ISA) += cc770_isa.o
 obj-$(CONFIG_CAN_CC770_PLATFORM) += cc770_platform.o
-
-ccflags-$(CONFIG_CAN_DEBUG_DEVICES) := -DDEBUG