jz4740-battery: Correct voltage change check

The check is supposed to avoid redundant update notifications, so it
should check for the difference between old and new voltage exceeding
a threshold.

Also make sure the result of a failed read is never stored.
Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

drivers/power/jz4740-battery.c

@@ -208,7 +208,7 @@ static void jz_battery_update(struct jz_battery *jz_battery)
 	}
 
 	voltage = jz_battery_read_voltage(jz_battery);
-	if (abs(voltage - jz_battery->voltage) < 50000) {
+	if (voltage >= 0 && abs(voltage - jz_battery->voltage) > 50000) {
 		jz_battery->voltage = voltage;
 		has_changed = true;
 	}