brcmfmac: Configure keep-alive packet on suspend

When entering suspend as a client station with wowlan enabled,
the Wi-Fi link is supposed to be maintained. In that state, no
more data is generated from client side, and the link stays idle
as long the station is suspended and as long the AP as no data to
transmit.

However, most of the APs kick-off such 'inactive' stations after
few minutes, causing unexpected disconnect (reconnect, etc...).

The usual way to prevent this is to submit a Null function frame
periodically as a keep-alive. This is something that can be host
/software generated (e.g. wpa_supplicant), but that needs to be
offloaded to the Wi-Fi controller in case of suspended host.

This change enables firmware generated keep-alive frames when
entering wowlan suspend, using the 'mkeep_alive' IOVAR.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1637596046-21651-1-git-send-email-loic.poulain@linaro.org