Fix testPhysicalEthernet failure for physical ethernet device

Other virtual interface ethernet test cases stop ethernet tethering
by deleting the virtual interface. To disable ethernet tethering for
testPhysicalEthernet, call stopTethering explicitly.

Bug: 271380398
Test: atest EtherentTetheringTest
Change-Id: I19a73f63227bab6a60d57d38b17a20a3f3f6c1d4