wlcore: Remove wl pointer from wl_sta structure

This field was added to wl_sta struct to get hw in situations
where it was not given to driver by mac80211. In our case,
get_expected_throughput op did not send hw to driver.

This patch reverts the change, as it is no longer needed due to commit
4fdbc67a ("mac80211: call get_expected_throughput only after adding
station") as hw is now sent as a parameter.

Signed-off-by: Maxim Altshul <maxim.altshul@ti.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>