thermal: exynos: Reading temperature makes sense only when TMU is turned on

When thermal sensor is not yet enabled, reading temperature might return
random value. This might even result in stopping system booting when such
temperature is higher than the critical value. Fix this by checking if TMU
has been actually enabled before reading the temperature.

This change fixes booting of Exynos4210-based board with TMU enabled (for
example Samsung Trats board), which was broken since v4.4 kernel release.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: 9e4249b4

 ("thermal: exynos: Fix first temperature read after registering sensor")
CC: stable@vger.kernel.org # v4.6+
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>