mac80211: driver can remain on channel if not using chan_ctx

Some of the drivers are not using channel context, but let the
stack to control/switch channels instead. For such cases, driver
can still remain on channel because the mac80211 stack actually
supports it.

The stack will check if the driver is using chan_ctx and has
ops->remain_on_channel been hooked. Otherwise it will start its
ROC work to remain on channel. So, even if the driver is not
using chan_ctx, the driver is still capable of doing remain on
channel.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Link: https://lore.kernel.org/r/20200312074337.16198-1-yhchuang@realtek.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>