iwlwifi: mvm: fix support for single antenna diversity

When the single antenna diversity support was sent upstream, only some
definitions were sent, due to a bad revert.

Fix this by adding the actual code.

Fixes: 5952e0ec ("iwlwifi: mvm: add support for single antenna diversity")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>