iwlwifi: mvm: fw: clean up hcmd struct creation

This is valid, but pretty uncommon in the driver, clean up
the code here a bit to use an initializer.

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.20220128153014.1717817392df.I75add2a50a69d28eaebfd67e5a0524bf43119a81@changeid


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