Fix: sessiond: wrong variable checked for error code

Coverity reports:
  1430513 Logically dead code

  The indicated dead code may have performed some action; that action
  will never occur.

  In ust_app_open_packets: Code can never be reached because of a
  logical contradiction (CWE-561)

The `open_ret` variable should be checked instead of `ret` after calling
consumer_open_channel_packets.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I49ad2d2b747e29414fee7f0727df1feb343695bb