powercap: Simplify powercap_init()

Simplify powercap_init() by reducing the number of redundant
assignments in it.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
[ rjw: Subject+changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>