(split) Add preliminary support for the Realtek RTL8188EUS and RTL8188ETV chipsets.

Committed over the TP-LINK TL-WN725N v2 (RTL8188EUS) on amd64 with WPA.
(split) Initialize rssi variable.
(split) Fix gcc build, initialize off variable.
(split) Split commit '867becd9' for the sake of subtree
1 of 2 (only in sys/dev/usb/wlan)

 Add preliminary support for the Realtek RTL8188EUS and RTL8188ETV chipsets.

 Committed over the TP-LINK TL-WN725N v2 (RTL8188EUS) on amd64 with WPA.
(split) Merge branch 'CURRENT' (commit 'b6e63d67')

Conflicts:
	sys/dev/usb/wlan/if_urtwn.c