mac80211: Shortcut minstrel_ht rate setup for non-MRR capable devices

Devices without multi rate retry support won't be able to use all rates
as specified by mintrel_ht. Hence, we can simply skip setting up further
rates as the devices will only use the first one.

Also add a special case for devices with only two possible tx rates. We
use sample_rate -> max_prob_rate for sampling and max_tp_rate ->
max_prob_rate by default.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>