net: tcp: Set the TCP sent flag after really sending pkt

Make sure that the sent flag is only set after we have really
sent the packet and the driver has verified that.
If the net_pkt_set_sent() is called while still in tcp.c, then
depending on how fast the device is, it might happen that the
retry timer expires before the packet is actually sent. This was
seen in frdm-k64f with ethernet and various debug prints activated.

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