thermal: exynos: replace threshold_falling check by Exynos SoC type one
Replace pdata->threshold_falling check for non-zero value in
exynos_tmu_initialize() by an explicit check for a SoC type
(all SoC types except Exynos5440 have pdata->threshold_falling
assigned to non-zero value in their struct exynos_tmu_registers
instances).
This is a preparation for introducing per-SoC type tmu_initialize
method.
There should be no functional changes caused by this patch.