drivers: thermal: step_wise: Clear mitigation on reaching clear threshold

Step wise algorithm will always determine the next mitigation action
based on the current operating level. When a cooling device is mitigated
by two rules simultaneously, each rule will have its own mitigation value.
When one rule goes below the clear threshold, step wise algorithm will
still decide to mitigate, since the next mitigation is determined based
on the current operating level. For rules, that doesn't have passive or
active polling, this mitigation action will stay till the rule triggers
the trip threshold for the next time.

In order to avoid this, during downward trend and when no throttling,
consider the last mitigation action sent from the rule. If it is already
at the lowest mitigation level, clear any mitigation.

Change-Id: I4ffd5733097393e5128260512b12576a14be8136
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>