exit/r8188eu: Replace the macro thread_exit with a simple return 0

exit/r8188eu: Replace the macro thread_exit with a simple return 0

The macro thread_exit is called is at the end of functions started
with kthread_run.  The code in kthread_run has arranged things so a
kernel thread can just return and do_exit will be called.

So just have rtw_cmd_thread and mp_xmit_packet_thread return instead
of calling complete_and_exit.

Link: https://lkml.kernel.org/r/20211020174406.17889-20-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>