iwlwifi: mvm: fix BT coex shared antenna activity check

The shared antenna should be forbidden to use only if there's
high BT activity. Comparing to BT_OFF was effectively causing
us to always forbid using the shared antenna for SISO. This
leads to degraded performance in scenarios where the shared
antenna would have better performance.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>