wifi: mac80211: Use struct instead of macro for PREP frame

wifi: mac80211: Use struct instead of macro for PREP frame

The existing PREP_IE_* macros access HWMP PREP frame fields via hardcoded
byte offsets. When the AE (Address Extension) flag is set, an additional
6 bytes appear mid-frame, making the offset arithmetic error-prone.

Introduce typed packed C structs to represent the PREP frame layout:
  - ieee80211_mesh_hwmp_prep_top: fixed fields before the optional AE
    address
  - ieee80211_mesh_hwmp_prep_bottom: fields after the optional AE address

Add ieee80211_mesh_hwmp_prep_get_bottom() to locate the bottom struct
correctly based on whether the AE flag is set.

This preparatory refactoring is needed to fix a 2-byte overread of
orig_addr in hwmp_prep_frame_process() when AE is enabled, which is
addressed in a subsequent patch.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Link: https://patch.msgid.link/20260529230952.124754-2-masashi.honma@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>