CLEANUP: stream_interface: cleanup loop information in si_conn_send_loop()

Though si_conn_send_loop() does not loop over ->snd_buf() after commit ed7f836,
there is still some codes left which use `while` but only execute once. This
commit does the cleanup job and rename si_conn_send_loop() to si_conn_send().

Signed-off-by: Godbach <nylzhaowei@gmail.com>