Input: MT - fix null pointer warning

Fixes the following sparse warning:
drivers/input/input-mt.c:138:40: warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <[e-mail]>
Signed-off-by: Henrik Rydberg <[e-mail]>

drivers/input/input-mt.c

···
  */
 void input_mt_report_pointer_emulation(struct input_dev *dev, bool use_count)
 {
-	struct input_mt_slot *oldest = 0;
+	struct input_mt_slot *oldest = NULL;
 	int oldid = dev->trkid;
 	int count = 0;
 	int i;