iwlwifi: fw api: make hdr a zero-size array again

In struct iwl_tx_cmd, there's no risk (as Arnd implied) that we
might access this as an array, as it's really not an array and
cannot be - there's only a single 802.11 header per frame. The
only reason for this member is for being able to access it a
bit more nicely.

On the other hand, this structure is used as a sub-struct in a
few places, and then some compilers (e.g. clang with certain
options) complain as you shouldn't have structs with variable-
length fields embedded in other structs.

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.20210205110447.46cd538c90bf.I92179567d96938598806b560be59d787c2a8cc16@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>