wifi: mac80211: add internal handler for wake_tx_queue

Start to align the TX handling to only use internal TX queues (iTXQs):

Provide a handler for drivers not having a custom wake_tx_queue
callback and update the documentation.

Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>