mac80211: call get_expected_throughput only after adding station

Depending on which method the driver implements, userspace could
call this (indirectly, by getting station info) before the driver
knows about the station, possibly causing it to misbehave.

Therefore, add a check for sta->uploaded which indicates that the
driver knows about the station.

Signed-off-by: Maxim Altshul <maxim.altshul@ti.com>
[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>