wifi: mac80211: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for on-stack definitions of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warnings:

net/mac80211/spectmgmt.c:151:47: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
net/mac80211/spectmgmt.c:155:48: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/Z-SQdHZljwAgIlp9@kspp


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