chnl: add chnl_port to get port from connected channel

[jalalmostafa]

This PR adds a new chnl_port API function, allowing us to know which port this channel is bound to.
It is important to allocate mbufs when sending data e.g.

pktmbuf_t* bufs[256];
int portid = chnl_port(cd);
pktdev_buf_alloc(portid, bufs, burst);
chnl_send(cd, bufs, burst);

This can be easily done as the PCB entry will be linked to a specific port (interface and queue) in the FIB when the connection has been established chnl_accept or chnl_connect has finished.

[KeithWiles]

Thank you