iwlwifi: support REDUCE_TX_POWER_CMD version 6

The new version of the command can support more subbands and CDB, so
it can contain more data than earlier versions.  Implement support for
the new version of the command, even though we don't have more data to
write to it yet.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200928121852.d709a8f17d1d.I9fa54883667c72dabf6d813c70be77538d9af38d@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>