net: Check error when pushing packet into stack

Make tcpip_input() to return 0 if there is an error when
processing the packet. Value != 0 indicates a successfull
packet processing and in this case the application will
free the packet.

Change-Id: I4aaeb5f0039cfbe25f7238a144964e31a8204148
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>