jz4740-battery: Fix signedness bug

wait_for_completion_interruptible_timeout() may return negative value.
In this case, checking if (t > 0)  will return true if t is unsigned.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>

drivers/power/jz4740-battery.c

@@ -67,7 +67,7 @@ static irqreturn_t jz_battery_irq_handler(int irq, void *devid)
 
 static long jz_battery_read_voltage(struct jz_battery *battery)
 {
-	unsigned long t;
+	long t;
 	unsigned long val;
 	long voltage;