charger-manager: Fix bug when check dropped voltage after fullbatt event

This patch check difference value between current voltage of battery and
desc->fullbatt_uV whether positve or negative number. If difference value
is negative number when current voltage of battery is larger than
desc->fullbatt_uV, charger-manager return immediately because battery is
fully charged.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

drivers/power/charger-manager.c

@@ -482,8 +482,9 @@ static void fullbatt_vchk(struct work_struct *work)
 		return;
 	}
 
-	diff = desc->fullbatt_uV;
-	diff -= batt_uV;
+	diff = desc->fullbatt_uV - batt_uV;
+	if (diff < 0)
+		return;
 
 	dev_info(cm->dev, "VBATT dropped %duV after full-batt.\n", diff);