iwlwifi: mvm: report per-chain signal to mac80211

Instead of reporting the maximum signal strength and the
antenna bitmap in the antenna field (which is really just
for radiotap and defined differently), report the signal
strength values per chain and set the chain bitmap.

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