drivers: thermal: of-thermal: notify thermal zone only if it is enabled

There is a case where of-thermal invokes thermal zone re-evaluation
for a zone even if it is disabled during sensor violation. It may
lead to thermal zone throttling even if that thermal zone is disabled.

Update of-thermal to re-evaluate thermal zone only if it is in
enabled state.

Change-Id: I0764d4719668231d27b4e264c46e5593653d90b3
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>