of: add 'const' to of_node_full_name parameter

As the function just returns the np->full_name or the string "<no-node>", the
passed device_node pointer is not changed in any way.

The passed parameter can therefore be a const pointer.

Also, fix the following error from checkpatch.pl:

ERROR: "foo* bar" should be "foo *bar"
+static inline const char* of_node_full_name(const struct device_node *np)

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>