[media] soc-camera: Fix a return value in case of error

If 'ov9640_reg_read()' does not return 0, then 'val' is left unmodified.
As it is not initialized either, the return value can be anything.

It is likely that returning the error code was expected here.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>