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>