Add a new inline helper function to ieee80211.h to
extract the disabled subchannels bitmap from an EHT
operation element, and use that in mac80211 where
we do that.

Link: https://msgid.link/20240129194108.d9f50dcec8d0.I8b08cbc2490a734fafcce0fa0fc328211ba6f10b@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>