iwlwifi: generic scan TX antenna forcing

In "iwlwifi: make scan antenna forcing more generic"
I introduced generic scan RX antenna forcing, which
here I rename to make it more evident. Also add scan
TX antenna forcing, since I will need that as well.

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