of: base, fix of_property_read_string_helper kernel-doc

It referenced of_property_read_string_util whereas the function name
is of_property_read_string_helper.

Introduced in a87fa1d81a9fb5e9adca9820e16008c40ad09f33 (of: Fix
overflow bug in string property parsing functions). Found out when
reviewing the stable 3.12 queue.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

drivers/of/base.c

@@ -1326,7 +1326,7 @@ int of_property_match_string(struct device_node *np, const char *propname,
 EXPORT_SYMBOL_GPL(of_property_match_string);
 
 /**
- * of_property_read_string_util() - Utility helper for parsing string properties
+ * of_property_read_string_helper() - Utility helper for parsing string properties
  * @np:		device node from which the property value is to be read.
  * @propname:	name of the property to be searched.
  * @out_strs:	output array of string pointers.