iwlwifi: mvm: rs - don't use the shared antenna when BT load is high

When we need only one antenna, we should refrain from using
the antenna that is shared with BT if BT load is high.
Fix this.

Reviewed-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>