iwlwifi: mvm: fix switch from shared antenna in case of BT load

Current code didn't handle well the case where we're in SISO using
ANT B and there's a BT load. Switch to ANT A in this case.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>