Correctly initialise the 2-chain antenna mask in the link quality table.

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