wifi: ieee80211: split EHT definitions out

The ieee80211.h file has gotten very long, continue splitting
it by putting EHT definitions into a separate file.

Link: https://patch.msgid.link/20251105153843.bf77fe169140.I691267e0edd914c604a5bfd447d33be00044c9b4@changeid


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