mtd-utils: Fix return value of ubiformat

This changeset fixes a feature regression in ubiformat.  Older versions of
ubiformat, when invoked with a flash-image, would return 0 in the case no error
was encountered.  Upon upgrading to latest, it was discovered that ubiformat
returned 255 even without encountering an error condition.

This changeset corrects the above issue and causes ubiformat, when given an
image file, to return 0 when no errors are detected.

Tested by running through my loading scripts and verifying ubiformat returned
0.

Signed-off-by: Barry Grussling <barry@grussling.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>