mac80211: fix supported rates IE if AP doesn't give us it's rates

If AP do not provide us supported rates before assiociation, send
all rates we are supporting instead of empty information element.

v1 -> v2: Add comment.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>