PM: core: Remove unnecessary (void *) conversions

Assignments from pointer variables of type (void *) do not require
explicit type casts, so remove such type cases from the code in
drivers/base/power/main.c where applicable.

Signed-off-by: Li zeming <zeming@nfschina.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>