The previous code simply hard-coded IWN_ANT_AB which is only correct for
some of the NICs.
Now, if the NIC is a 1-stream TX, you need to set IWN_ANT_AB and _not_
just a single antenna. The Intel 5100 firmware panics the moment the
link quality table is updated.
So!
* no secondary antenna? Set it to IWN_ANT_AB;
* two-stream device? Transmit on the full transmit antenna configuration.
Tested:
* Intel 5100, STA
* Intel 2200 (eadler)
Obtained from: Linux iwlwifi