Do as we have done for iwlwifi (f808c43ad9, bee60c9897) add a
completion event for device registration which calls into 802.11
and creates the wifi "device" (net80211 com instance).
This is needed as otherwise the deferred work in the mt76 drivers
(mt7915, mt7921, mt7925, mt7996; but not the 7615 [*]) would make
driver loading return before the wifi device is there. We would then
continue, e.g., during rc startup and race possibly trying to create
a vap (wlan interface) with the underlying device not being registered
yet and fail.
[*] the 7615 does not seem to do this asynchronously so is fine.
Sponsored by: The FreeBSD Foundation
Tested on: 7921, others to be tested at time
MFC after: 3 days
Bjoern A. Zeeb
baf8561bdc