UBI: return on error in rename_volumes()

I noticed this during a code review.  We are checking that the strlen()
of ->name is not less than the ->name_len which the user gave us.  I
believe this bug is harmless but clearly we meant to return here instead
of setting an error code and then not using it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>