bearer-qmi: use complete_connect() in in all steps of the attempt

The connect_cleanup() method takes care of cleaning up the ongoing connect cancellables stored in the MMBearerQmi private data. Not using it would lead to assert failures when disposing the bearer object.

  0x00007ca75b4dea3f (libc.so.6 - pthread_kill.c: 44)     __pthread_kill_implementation
  0x00007ca75b493c6c (libc.so.6 - raise.c: 26)    raise
  0x00007ca75b47f462 (libc.so.6 - abort.c: 79)    abort
  0x00007ca75bca4688 (libglib-2.0.so.0 - gtestutils.c: 3450)      g_assertion_message
  0x00007ca75bca46fc (libglib-2.0.so.0 - gtestutils.c: 3476)      g_assertion_message_expr
  0x00005785d2f5aec4 (ModemManager - mm-bearer-qmi.c: 2974)       dispose
  0x00007ca75ba4cd0b (libgobject-2.0.so.0 - gobject.c: 3891)      g_object_unref
  0x00007ca75bc7c834 (libglib-2.0.so.0 - glist.c: 1092)   g_list_foreach
  0x00007ca75bc7c834 (libglib-2.0.so.0 - glist.c: 246)    g_list_free_full
  0x00005785d2f071a7 (ModemManager - mm-bearer-list.c: 456)       dispose
  0x00007ca75ba4cd0b (libgobject-2.0.so.0 - gobject.c: 3891)      g_object_unref
  0x00005785d2f0cf5f (ModemManager - mm-broadband-modem.c: 13420) dispose
  0x00007ca75ba4cb6f (libgobject-2.0.so.0 - gobject.c: 1448)      g_object_run_dispose
  0x00005785d2f20ae1 (ModemManager - mm-device.c: 418)    clear_modem
  0x00005785d2f20d9d (ModemManager - mm-device.c: 461)    modem_valid
  0x00007ca75ba4820e (libgobject-2.0.so.0 - gclosure.c: 832)      g_closure_invoke
  0x00007ca75ba5cd00 (libgobject-2.0.so.0 - gsignal.c: 3812)      signal_emit_unlocked_R
  0x00007ca75ba5dd8b (libgobject-2.0.so.0 - gsignal.c: 3565)      g_signal_emit_valist
  0x00007ca75ba5e1d5 (libgobject-2.0.so.0 - gsignal.c: 3622)      g_signal_emit
  0x00007ca75ba51c6a (libgobject-2.0.so.0 - gobject.c: 1428)      g_object_dispatch_properties_changed
  0x00007ca75ba4d159 (libgobject-2.0.so.0 - gobject.c: 1552)      g_object_notify_by_spec_internal
  0x00005785d2efdf62 (ModemManager - mm-base-modem.c: 1783)       base_modem_invalid_idle
  0x00007ca75bc807a4 (libglib-2.0.so.0 - gmain.c: 3460)   g_main_dispatch
  0x00007ca75bc807a4 (libglib-2.0.so.0 - gmain.c: 4200)   g_main_context_dispatch
  0x00007ca75bc80ac3 (libglib-2.0.so.0 - gmain.c: 4276)   g_main_context_iterate
  0x00007ca75bc80d41 (libglib-2.0.so.0 - gmain.c: 4479)   g_main_loop_run
  0x00005785d2ef07df (ModemManager - main.c: 236) main
  0x00007ca75b47f6c5 (libc.so.6 - libc_start_call_main.h: 58)     __libc_start_call_main
  0x00007ca75b47f781 (libc.so.6 - libc-start.c: 389)      __libc_start_main_impl
  0x00005785d2ef0130 (ModemManager + 0x000b1130)  _start
  0x00007ffd10207c37