mfd: ab8500-debugfs: Use NULL to initialise remaining NULL pointer

Partly for coding style reasons, but mostly because sparse warns on it.

This patch is a completion of a previous patch by Mark Brown.
Reviewed-by: NMark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/mfd/ab8500-debugfs.c

@@ -60,7 +60,7 @@ struct ab8500_prcmu_ranges {
 static struct ab8500_prcmu_ranges debug_ranges[AB8500_NUM_BANKS] = {
 	[0x0] = {
 		.num_ranges = 0,
-		.range = 0,
+		.range = NULL,
 	},
 	[AB8500_SYS_CTRL1_BLOCK] = {
 		.num_ranges = 3,