Temp disable Tethering if DUN is required

Getting source-based routing working is too risk for this point
in the project but tethering is broken otherwise, so disable
the tethering option if DUN is required until we can get a real
fix in.

bug:5495862
Change-Id: I5e852bf30c887599024a8b61af86ffec1d5333af
1 file changed