wifi: mac80211: remove misleading j_0 construction parts

wifi: mac80211: remove misleading j_0 construction parts

The GCM algorithm implementation in the kernel assumes that
a 12-byte IV is passed, not the actual J_0 from the GCM spec.
Don't rename, that'd be messy, but also don't fill the bytes
beyond the IV that aren't used, since otherwise it looks as
though j_0[12] is used uninitialized.

Link: https://patch.msgid.link/20241021151414.798ceb7a5896.Ic57751edad228d56865ecf7433fef469e5e0a4aa@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>