rt2x00: Limit TX done looping to number of TX ring entries

Similar to rt2800pci, remove the check for duplicate
register reading, and instead limit the for-loop to
the maximum number of TX entries inside a queue.
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Acked-by: NGertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt61pci.c

@@ -2052,29 +2052,24 @@ static void rt61pci_txdone(struct rt2x00_dev *rt2x00dev)
 	struct txdone_entry_desc txdesc;
 	u32 word;
 	u32 reg;
-	u32 old_reg;
 	int type;
 	int index;
+	int i;
 
 	/*
-	 * During each loop we will compare the freshly read
-	 * STA_CSR4 register value with the value read from
-	 * the previous loop. If the 2 values are equal then
-	 * we should stop processing because the chance is
-	 * quite big that the device has been unplugged and
-	 * we risk going into an endless loop.
+	 * TX_STA_FIFO is a stack of X entries, hence read TX_STA_FIFO
+	 * at most X times and also stop processing once the TX_STA_FIFO_VALID
+	 * flag is not set anymore.
+	 *
+	 * The legacy drivers use X=TX_RING_SIZE but state in a comment
+	 * that the TX_STA_FIFO stack has a size of 16. We stick to our
+	 * tx ring size for now.
 	 */
-	old_reg = 0;
-
-	while (1) {
+	for (i = 0; i < TX_ENTRIES; i++) {
 		rt2x00pci_register_read(rt2x00dev, STA_CSR4, &reg);
 		if (!rt2x00_get_field32(reg, STA_CSR4_VALID))
 			break;
 
-		if (old_reg == reg)
-			break;
-		old_reg = reg;
-
 		/*
 		 * Skip this entry when it contains an invalid
 		 * queue identication number.