wcn36xx: Enable firmware offloaded keepalive

This patch calls wcn36xx_smd_keep_alive_req() on the STA patch immediately
after associating with an AP.

This will cause the firmware to send a NULL packet out to the AP every 30
seconds, thus offloading keep-alive processing from the SoC to the
firmware.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201103121735.291324-4-bryan.odonoghue@linaro.org