This fixes a regression in __of_find_path() for flash devices created by
the cadence-quadspi driver, which do not have 'dev->driver' set.  Such
devices do not have 'dev->bus' set either, so we can use that to qualify
the existing test.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>