rt2x00: do not generate seqno in h/w if QOS is disabled

This is workaround H/W or F/W bug, see in code comments. Without the fix
ping can receive duplicated ICMP frames while associated with legacy AP.

Reported-by: Walter Goldens <goldenstranger@yahoo.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00.h

@@ -693,6 +693,7 @@ enum rt2x00_state_flags {
 	CONFIG_CHANNEL_HT40,
 	CONFIG_POWERSAVING,
 	CONFIG_HT_DISABLED,
+	CONFIG_QOS_DISABLED,
 
 	/*
 	 * Mark we currently are sequentially reading TX_STA_FIFO register