iwlwifi: mvm: support new TX power command

Add support for the new (bigger) TX power command. This doesn't
actually take advantage of the new capabilities (to set per-chain
TX power limits) but makes the code compatible with newer firmware
images expecting the larger command.

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