mwl8k: Tell mac80211 we have rate adaptation in FW

All mwl8k parts perform rate control in firmware.  Make this known to
mac80211 so that it does not launch minstrel.  Also, because actual tx
rate information is not available from the firmware, invalidate the
rate status before returning the skb to mac80211.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>