drivers: thermal: disable the mitigation when zone is disabled

For zones that are disabled, the get temperature can read a valid
temperature for sensors using of-thermal interface. This can lead
to a case where a previous mitigation on a disabled zone can be
still active.

To avoid this, read an invalid temperature value when a zone is
disabled. The invalid value is actually a very high or low temperature
based on whether the zone is tracking low or high threshold. Also clear
any previous mitigation on the cooling devices when a thermal zone is
disabled.

Change-Id: I4e4e725c004297f97625797639229303b1b7d9cd
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>