wifi: mac80211: status: look up band only where needed

For MLD, we might eventually not really know the band on status,
but some code assumes it's there. Move the sband lookup deep to
the code that actually needs it, to make it clear where exactly
it's needed and for what purposes.

For rate control, at least initially we won't support it in MLO,
so that won't be an issue.

For TX monitoring, we may have to elide the rate and/or rely on
ieee80211_tx_status_ext() for rate information.

This also simplifies the function prototypes.

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