tether: tethering chooses wrong upstream when mobile_dun is required

When mobile_dun is required, the default mobile type will
be deleted from upstream list, this makes wifi become
the highest priority upstream even whne wifi is not available.

Change-Id: I2c12f6cd6d52331220dce900609ad072f9ec3fbc
CRs-fixed: 671675
(cherry picked from commit 43d14a4f)