wifi: iwlwifi: dvm: clean up rs_get_rate() logic

wifi: iwlwifi: dvm: clean up rs_get_rate() logic

The logic here is rather convoluted - we cannot get here with
lq_sta being NULL as mac80211 will (no longer) call us like
that, and since I removed the rate_control_send_low() call in
this function there's no longer any point in setting priv_sta
to NULL either.

So the only thing that remains to check is if we have actually
initialized our lq_sta->drv pointer, and exit if we didn't in
which case we'll use the data mac80211 already set up for the
low rate usage.

Reviewed-by: Luciano Coelho <luciano.coelho@intel.com>
Link: https://msgid.link/20240510170500.a4cdb41825eb.Id202bcc967c32829f70ab1412f8893b6eb7f78e2@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>