exit/rtl8712: Replace the macro thread_exit with a simple return 0

exit/rtl8712: Replace the macro thread_exit with a simple return 0

The macro thread_exit is called is at the end of a function 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 the cmd_thread return instead of calling complete_and_exit.

Link: https://lkml.kernel.org/r/20211020174406.17889-19-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>