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.

Cc: Amit Daniel Kachhap <amit.daniel@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>