Return WiFi data usage for WiFi-only devices

Fixes "0B of data used" in "Data Usage" summary highlight
in Settings if Telephony is not present or turned off.

Reported-By: L.W Reek <syphyr@gmail.com>
Refactored-By: Michael W <baddaemon87@gmail.com>
Signed-off-by: Vasyl Gello <vasek.gello@gmail.com>

Change-Id: I9623054c2e6e70060e65e93b18a11e8fb281d5d7