Create a common structure to contain all different versions of the
tx_power_cmd instead of making a union of the different structs
everywhere we need them.  Also move the common part of these structs
into a separate structure (instead of reusing v3) and leave the
per_chain_restriction part out of the common part, because this will
change in version 6 of the command (which will be added soon).

While at it, rename per_chain_restriction to per_chain to shorten it.
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200928121852.4f0bea9fe077.Ib3b540a8288af32d6fa213448e13f82763f85bc9@changeidSigned-off-by: Luca Coelho <luciano.coelho@intel.com>