thermal: exynos: Miscellaneous fixes to support falling threshold interrupt

Below fixes are done to support falling threshold interrupt,
* Falling interrupt status macro corrected according to exynos5 data sheet.
* The get trend function modified to calculate trip temperature correctly.
* The clearing of interrupt status in the isr is now done after handling
  the event that caused the interrupt.

Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>