iwlwifi: make scan antenna forcing more generic

Some future hardware will also require some antenna
overrides so make the current logic more generic;
right now it is semantically based on a workaround
for off-channel reception but the reasons for the
new antenna overrides will be different.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>