net: context: Set target network interface in send if needed

If we are sending a network packet and if the remote address
is not set in the context (which means that connect() has not
been called), then we must set the target network interface
to a proper value.
This is done so that when we select the local source address,
we might select the wrong interface if we have multiple network
interfaces in the system. In this case the packet would be always
assigned to first network interface regardless of the destination
address.

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