wifi: ieee80211: split mesh definitions out
The ieee80211.h file has gotten very long, start splitting it
by putting mesh definitions into a separate file.
Link: https://patch.msgid.link/20251105153843.489713ca8b34.I3befb4bf6ace0315758a1794224ddd18c4652e32@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>