When the flash doesn't hold a valid firmware image there are still
some things (like accessing the flash) possible. Don't return an error
code in this case but return 0 instead. This way the driver is
regularily bound and just doesn't provide a network interface.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>