Move the error check for "brctl addif"

The git commit 873aad92944f8840e772d65eda4b3320d65a9ce7[1] has moved
"brctl addif" command, but the commit did not move its error check.

Now the error check sees the result of "ip addr show dev <interface>
scope global" command wrongly, and the command outputs "Dump was
interrupted and may be inconsistent" sometimes because of a kernel change.
As the result, gate failures happen sometimes.

This patch moves the error check to right place.

[1]: https://github.com/openstack/nova/commit/873aad92944f8840e772d65eda4b3320d

Change-Id: I92ee96e3aa34f7cd154a8ce4b33852b96cabe8c9
Closes-Bug: #1334142
Merge "Move the error check for "brctl addif""