wifi: mac80211: fix race condition on enabling fast-xmit

fast-xmit must only be enabled after the sta has been uploaded to the driver,
otherwise it could end up passing the not-yet-uploaded sta via drv_tx calls
to the driver, leading to potential crashes because of uninitialized drv_priv
data.
Add a missing sta->uploaded check and re-check fast xmit after inserting a sta.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://msgid.link/20240104181059.84032-1-nbd@nbd.name


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