Tether: fix bt-pan and wlan0 timing issue

In a situation where bt-pan or wlan
interface comes up later than the tether
command, tethering will fail with
interface not found error. This can
happen when the wifi hotspot is quickly
and repeatedly enabled and disabled.
Fix is to ignore interface down for
these cases

Change-Id: I464b87f1a6470401b2ef4e7507b804b1da5b5af2
CRs-Fixed: 784379