HID: wacom: Assign boolean values to a bool variable

Fix the following coccicheck warnings:

./drivers/hid/wacom_wac.c:2536:2-6: WARNING: Assignment of
0/1 to bool variable.
Reported-by: NAbaci Robot <abaci@linux.alibaba.com>
Signed-off-by: NJiapeng Zhong <abaci-bugfix@linux.alibaba.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_wac.c

@@ -2533,7 +2533,7 @@ static void wacom_wac_finger_slot(struct wacom_wac *wacom_wac,
 	    !wacom_wac->shared->is_touch_on) {
 		if (!wacom_wac->shared->touch_down)
 			return;
-		prox = 0;
+		prox = false;
 	}
 
 	wacom_wac->hid_data.num_received++;