cfg80211: Validate legacy rateset.

Legacy rates are not validated while configuring
tx rateset using iw. So below cmd is accepted by nl80211.
sudo iw wlan2 set bitrates legacy-2.4 1 2 3

Validate legacy rates and return
error if any rate in the rateset is not valid.

Signed-off-by: Bala Shanmugam <bkamatch@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>