UBI: silence a warning

This patch silences the following warning :

  drivers/mtd/ubi/vmt.c:73: warning: 'ret' may be used uninitialized in this function

gcc can't see that we always initialize ret in all situations where it is
actually used. The one case where it's not initialized is when we BUG(),
but gcc doesn't know that we won't then continue and use an uninitialized
'ret'.

This patch results in code that does exactely the same as before, but it
also makes gcc shut up, so we generate one less line of warning noise.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>