iwlwifi: mvm: implement CSI reporting

Implement CSI (channel estimation matrix) reporting in the mvm
driver, if the firmware has the capability.

Currently only a debugfs API is provided as the API is still
under discussion.

For now, RX aggregation must be disabled to use this feature
on data frames as we haven't found a good way to attach the
data to A-MPDUs, given complexities with multi-queue.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>