mac80211: Add ieee80211_hw pointer to get_expected_throughput

The variable is added to allow the driver an easy access to
it's own hw->priv when the op is invoked.

This fixes a crash in wlcore because it was relying on a
station pointer that wasn't initialized yet. It's the wrong
way to fix the crash, but it solves the problem for now and
it does make sense to have the hw pointer here.

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