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>