mwl8k: Check outgoing rate for a station to decide if ampdu can be created

If the outgoing packet rate to a particular HT station is <=6.5
Mbps, do not attempt to create an ampdu. Also, if the outgoing
rate is legacy rate, do not create an ampdu.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>