wlcore: Add support for get_expected_throughput opcode

Adding this opcode, allows the TI wireless driver,
to report throughput directly from FW to mac80211.

This is used mainly for mesh metric calculation.

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