wifi: mac80211: spectmgmt: simplify 6 GHz HE/EHT handling

Clean up the code here a bit to have only a single call
to ieee80211_chandef_he_6ghz_oper() by using a local
pointer variable for the difference.

Link: https://msgid.link/20240312112048.94c421d767f9.Ia7ca2f315b392c74d39b44fa9eb872a2e62e75c1@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>