wifi: mac80211: clean up STA NSS handling

wifi: mac80211: clean up STA NSS handling

Move ieee80211_sta_init_nss() from VHT code to station code,
and disentangle it from rate control. This way, it becomes
clearer when 'rx_nss' is set up.

While doing this, fix the client side code to set up
link_sta->op_mode_nss instead of link_sta->pub->rx_nss for
the opmode element in association response, and remove the
(now wrong) comment about handling that in the function.

type=cleanup
ticket=none

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-on: https://gerritwcs.ir.intel.com/c/iwlwifi-stack-dev/+/273695
Tested-by: iil_jenkins iil_jenkins <EC.GER.UNIX.IIL.JENKINS@INTEL.COM>
tested: iil_jenkins iil_jenkins <EC.GER.UNIX.IIL.JENKINS@INTEL.COM>
Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20260415144514.bef3d07beeb6.I97fb93ccc1b366110ab23de58fcd73676cdd85d6@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>