adb: fix killing of old version of adb process

The original code was:

  if (strcmp(__adb_error, "unknown host service") != 0)

But that was changed by 078f0fcf to:

  if (*error == "unknown host service") {

I think the comparison should be != so that "unknown host service"
falls-through and kills the server, and so if it is some other error,
that the other error is returned immediately.

Change-Id: Ia490a4a870d1d123a3c5ab258dd5fa0930e8032d
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>