net: dns: Check and parse received mDNS responses properly

The mDNS packet receive had issues:

* The DNS id needs to be 0 for both sending and receiving, we did
  not accepted 0 incoming id.
* The mDNS response does not have any questions in it so we just
  need to skip the question count checks in response.
* Skip the Cache-Flush bit in Class field so that we can properly
  parse CLASS_IN value.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>