PM: sleep: spread "const char *" correctness

Fixed string literals can be referred to as "const char *".

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
[ rjw: Minor subject edit ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
diff --git a/kernel/power/power.h b/kernel/power/power.h
index ba2094d..32fc89a 100644
--- a/kernel/power/power.h
+++ b/kernel/power/power.h
@@ -32,7 +32,7 @@ static inline int init_header_complete(struct swsusp_info *info)
 	return arch_hibernation_header_save(info, MAX_ARCH_HEADER_SIZE);
 }
 
-static inline char *check_image_kernel(struct swsusp_info *info)
+static inline const char *check_image_kernel(struct swsusp_info *info)
 {
 	return arch_hibernation_header_restore(info) ?
 			"architecture specific data" : NULL;