UBI: Clean up return in ubi_remove_volume()

My static checker says that "err" can be uninitialized if
"vol->reserved_pebs" is <= 0.  I don't think that can happen but
returning a literal is cleaner anyway.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>