thermal: exynos: remove needless therm_trip_en_shift abstraction

reg->therm_trip_en_shift is used only in exynos_tmu_initialize()
and not accessed on Exynos4210 (also reg->therm_trip_en_shift is
not even assigned in exynos4210_tmu_registers but it is assigned
to identical value for all other SoC types) so the register
abstraction is not needed and can be removed.

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>