thermal: exynos: fix: Check if data->tmu_read callback is present before read

The exynos_tmu_data() function should on entrance test not only for valid
data pointer, but also for data->tmu_read one.
It is important, since afterwards it is dereferenced to get temperature code.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>