mtd: always initialize retlen to zero

Make sure that the retlen is set to 0 in case of error. This harmonizes
drivers - some set it to 0 in some error cases and do not write anything
in other error cases. Now we can do this consistently for all drivers.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>