iwlwifi: mvm: add DCM flag to rate pretty-print

It's useful to know if DCM was enabled, add this flag
to the rate pretty-printer.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200417131727.891bb9741eca.Ia66448f7e00be9e4c9ea7147b90d4fcd5f1d3845@changeid