net: Yield before sending packets

Using net_receive with long timeouts breaks net_reply. Sending a lot of
packets in a row without yielding causes failures since the network
stack can't send them. Add yield code for both net_reply and net_send.

Jira: ZEP-469
Jira: ZEP-497

Change-Id: Ic59769462e0575da0898d35745d49647f7e43edd
Signed-off-by: Jaakko Hannikainen <jaakko.hannikainen@intel.com>