cpu: clean up register_cpu func

This patch could reduce one branch in this function. Also
make the code more readble.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
To: linux-kernel@vger.kernel.org
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-pm@vger.kernel.org
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>