wifi: ipw2x00: fix array of flexible structures warnings

There are a number of these here, fix them by using
appropriate casts. No binary changes.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220904212910.645346411660.I471e8fadce54ea262920828f25b8e84545bcd07e@changeid