wifi: wfx: implement wfx_remain_on_channel()

With some conditions, the device is able to send/receive frames during
scan operation. So, it is possible to use it implement the "remain on
channel" feature. We just ask for a passive scan (without sending any
probe request) on one channel.

This architecture allows to leverage some interesting features:
  - if the device is AP, the device switches channel just after the next
    beacon and the beacons are stopped during the off-channel interval.
  - if the device is connected, it advertises it is asleep before to
    switch channel (so the AP should stop to try to send data)

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231004172843.195332-9-jerome.pouiller@silabs.com