of: fix CONFIG_OF=n prototype of of_node_full_name()

Make the CONFIG_OF=n prototpe of of_node_full_name() mateh the CONFIG_OF=y
version.

Fixes compile warnings like this:

sound/soc/soc-core.c: In function 'soc_check_aux_dev':
sound/soc/soc-core.c:1667:3: warning: passing argument 1 of 'of_node_full_name' discards 'const' qualifier from pointer target type [enabled by default]
   codecname = of_node_full_name(aux_dev->codec_of_node);

when CONFIG_OF is not defined.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Grant Likely <grant.likely@linaro.org>