of: fix handling of '/' in options for of_find_node_by_path()

Ensure proper handling of paths with appended options (after ':'),
where those options may contain a '/'.

Fixes: 7914a7c5

 ("of: support passing console options with stdout-path")
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Cc: <stable@vger.kernel.org> # 3.19
Signed-off-by: Rob Herring <robh@kernel.org>