wifi: mac80211: optionally pass chandef to ieee80211_sta_cap_rx_bw()

We'll need this function to take a new chandef in
(some) channel switching cases, so prepare for that
by allowing that to be passed and using it if so.
Clean up the code a little bit while at it.

Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20240612143418.772313f08b6a.If9708249e5870671e745d4c2b02e03b25092bea3@changeid


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