Add support for MIMO Tx rates (MCS 8-15) to iwm(4).

The nominal maximum Tx rate is now 144Mbit/s (MCS 15, 20MHz channel, SGI).
In practice it's more around 30Mbit/s, though. The most significant limiting
factor is probably lack of Tx aggregation support. (Due to protocol overhead,
reaching the nominal max is pretty much impossible anyway. When shopping wifi
devices, do not believe what stickers on the box are saying; full of lies!)

But APs will send us aggregates, so Rx should be faster than Tx (which was
already the case, just less pronounced).

Tested by phessler@, bmercer@, tb@
ok tb@

0 files changed, 0 insertions, 0 deletions