iwlwifi: mvm: fix RFH config command with >=10 CPUs

If we have >=10 (logical) CPUs, our command size exceeds the
internal buffer size and the command fails; fix that by using
IWL_HCMD_DFL_NOCOPY for the command that's allocated anyway.

While at it, also fix the leak of cmd, and use struct_size()
to calculate its size.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Fixes: 8edbfaa1

 ("iwlwifi: mvm: configure multi RX queue")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>