[iwm] fix default antenna selection a bit; fix 5GHz rate control handling.

• Don't do the antenna switching when setting up the rate table - we don't take into account whether it's an active antenna or not (eg shared with BT.) I'll look into this a bit more later.

• The default antenna is still 1, I'll look into that a bit more later. (So no, this doesn't fix it for Larry who needs ANT_B to be active, not ANT_A.)

• My changes to the rate control setup used the wrong method for finding a suitable rate, which led to 1M CCK frames being queued for 11a operation. This is .. sub-optimal. Change the rate control lookup path to use the global table instead of the per-node table, which won't be setup until the node rate table is setup (which happens way too late in this game.)

Tested:

• Intel 7260, 2G and 5G operation.