thermal: exynos: Fix wrong control of power down detection mode for Exynos7

This patch fixes the wrong control of PD_DET_EN (power down detection mode)
for Exynos7 because exynos7_tmu_control() always enables the power down detection
mode regardless 'on' parameter.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>