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 a87fa1d8 (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>