thermal: gov_step_wise: Clean up thermal_zone_trip_update()

Do some assorted cleanups in thermal_zone_trip_update():

 * Compute the trend value upfront.
 * Move old_target definition to the block where it is used.
 * Adjust white space around diagnostic messages and locking.
 * Use suitable field formatting in a message to avoid an explicit
   cast to int.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>