drivers: thermal: Avoid trip evaluation for disabled thermal zone

Avoid trip evaluation for disabled thermal zone, as the temperature
reading will be invalid and using the same to evaluate the trips will
lead to undefined behavior.

Change-Id: I5ea744d36875a425b0c3ed07c3ab45c94f9b59ca
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>