iwlwifi: use antenna A only under high BT load

When bluetooth indicated high load, we should use
only antenna A in 2.4 GHz for management frames.
Add this condition to iwl_toggle_tx_ant() to make
sure it'll always be met. Note that scanning has
a separate way of forcing the antenna, because we
should scan on antenna A only regardless of BT
traffic load.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>