Fix: lttng-disable-event: erroneous error code used in logging

66cefebdc introduced `disable_ret` to hold the integral result of
lttng_disable_event_ext() instead of mixing enums and integers.

In doing so, one use site was left unmodified which resulted in the
wrong variable being used to log an error when the command failed.

This was spotted by clang:
  commands/disable_events.cpp:252:21: warning: result of comparison of
  constant -83 with expression of type 'enum   cmd_error_code' is always false [-Wtautological-constant-out-of-range-compare]
                                      command_ret == -LTTNG_ERR_NEED_CHANNEL_NAME ?

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I282ebee0140f7044847aab6c43ba6bd0c185fc46