net: tcp2: Update tcp_pkt_alloc() macros

In order to use struct net_pkt for the outgoing data queue,
update tcp_pkt_alloc() macros, so they support allocation
without buffer.

Additionally, merge tcp_pkt_alloc() macros macros into one.

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