net: tcp: do not drop successfully received packets

Each time a successfully TCP connection is done, the number of dropped
TCP packets increases by 2. This is happens because when receiving an
initial SYN packet, or an ACK packet following a SYN+ACK packet,
NET_DROP is returned even if there is no error.

Fix that by replacing the two corresponding "return NET_DROP" by
"net_pkt_unref(pkt)" followed by "return 0".

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>