We have two functions which do the same thing. Standardise on
dev_has_ofnode() since there is no such thing as an 'invalid' ofnode in
normal operation: it is either null or missing.
Also move the functions into one place.
Signed-off-by: Simon Glass <sjg@chromium.org> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>