drivers: thermal: Update mitigation state reading logic from devicetree

Thermal cooling device uses THERMAL_MAX_LIMIT macro so that users can
define any mitigation state as an offset from max supported state for
that cooling device. But the logic to get actual state from this
offset config ignores if devicetree cooling device state is
THERMAL_MAX_LIMIT minus max supported state for that cooling device.

Update mitigation state calculation logic not accounted correctly.

Change-Id: I8fbdbf19e44e4bfbac315afe4cf79d40501bda58
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>