net: context: TCP packets must be send via net_tcp_send_data()

Do not shortcut the sending side for TCP as we need to use
net_tcp_send_data() to send TCP data in order not to have
double unref in Bluetooth or IEEE 802.15.4 technologies which
has a special TCP packet sending logic.

Change-Id: I077db336c6335ccdbbafa4600f98388ecf33955f
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>