power: Use subdir-ccflags-* to inherit debug flag

Use subdir-ccflags-* instead of ccflags-* to inherit the debug
settings from Kconfig when traversing subdirectories.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

drivers/power/Makefile

-ccflags-$(CONFIG_POWER_SUPPLY_DEBUG) := -DDEBUG
+subdir-ccflags-$(CONFIG_POWER_SUPPLY_DEBUG) := -DDEBUG
 
 power_supply-y				:= power_supply_core.o
 power_supply-$(CONFIG_SYSFS)		+= power_supply_sysfs.o