PM / wakeup: wakeup_source_create: use kstrdup_const

Using kstrdup_const allows us to save a little runtime memory (and a
string copy) in the common case where name is a string literal.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>