wifi: mwifiex: fix array of flexible structures warnings

There are two, just change them to have a "u8 data[]" type
member, and add casts where needed. No binary changes.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220904212910.2c885310ebee.If7177ea588b56c405eee6e6df595e9efccdfb99a@changeid