staging: wfx: fix counter overflow

Some weird behaviors were observed when connection is really good and
packets are small. It appears that sometime, number of packets in queues
can exceed 255 and generate an overflow in field usage_count.
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-4-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/data_tx.h

@@ -39,8 +39,8 @@ struct wfx_link_entry {
 
 struct tx_policy {
 	struct list_head link;
+	int usage_count;
 	u8 rates[12];
-	u8 usage_count;
 	u8 uploaded;
 };