thermal: gov_step_wise: Simplify get_target_state()

The step-wise governor's get_target_state() function contains redundant
braces, redundant parens and a redundant next_target local variable, so
get rid of all that stuff.

No intentional functional impact.

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