linux-user: Return success from m68k set_thread_area syscall

The m68k set_thread_area syscall implementation failed to set the
return value. Correctly set it zero, since this syscall will always
succeed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1375093909-13653-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

linux-user/syscall.c

@@ -8523,6 +8523,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
       {
           TaskState *ts = ((CPUArchState *)cpu_env)->opaque;
           ts->tp_value = arg1;
+          ret = 0;
           break;
       }
 #else