wifi: mac80211: adjust EHT capa when lowering bandwidth

If intending to associate with a lower bandwidth, remove capabilities
related to 320 MHz from the EHT capabilities element. Also change the
EHT MCS-NSS set accordingly: if just reducing 320->160 or similar the
format doesn't change, just cut off the last bytes. If changing from
higher bandwidth to 20 MHz only EHT STA, adjust the format.

Note that this also requires adjusting the caller in mlme.c since the
data written can now be shorter than it determined. We need to clean
all that up. Since the other callers pass NULL for the conn limit, we
don't need to change things there.

Link: https://msgid.link/20240129202041.b5f6df108c77.I0d8ea04079c61cb3744cc88625eeaf0d4776dc2b@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>