net: Check that net context is unset based on IP protocol

Rather than checking whether the remote port is zero, use the IP protocol
field. The remote port might be zero if all incoming connections will be
accepted.

Change-Id: Ia1f24ae94fab27c561267c7bbc0f709fe98ee7e8
Signed-off-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>