realtekwifi/rtwn: Synchronize with FreeBSD trunk as of today.

A few changes ommitted (e.g. NET_EPOCH) as they are not needed.
Adds support for a variety of USB devices and one PCI device.

realtekwifi: Sync with FreeBSD trunk.

This includes changes to support the 8188EE device, as well as a
number of critical bugfixes to 8192EE devices, which should make
the driver much more stable and reliable overall.

realtekwifi: Upgrade to FreeBSD 12.

This driver was completely rewritten and merged with its USB counterpart
in FreeBSD 12. I've imported the USB code, even though it's not compiled
in at all, because it's more than likely I'll get around to USB WiFi support
before FreeBSD 13 comes...

Untested, because I don't have one of these PCI devices.