mac80211: Inform AP when returning operating channel

Because we can miss AP wakeup (beacon) while scanning other channels,
it's better go into wakeup state and inform the AP of that upon
returning to the operating channel, rather than staying asleep and
waiting for the next TIM indicating traffic for us.

This saves precious time, especially when we only have 200ms inter-
scan period for monitoring the active connection.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Link: https://lore.kernel.org/r/1593420923-26668-1-git-send-email-loic.poulain@linaro.org
[rewrite commit message a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>