UBI: init_volumes: Ignore volumes with no LEBs

UBI assumes that ubi_attach_info will only contain ubi_ainf_volume
structures for volumes with at least one LEB.
In scanning mode this is true because UBI can nicely create a ubi_ainf_volume
on demand while creating the EBA table.

For fastmap this is not true, the fastmap on-flash structure has a list of
all volumes, the ubi_ainf_volume structures are created from this list.
So it can happen that an empty volume ends up in init_volumes().

We can easely deal with that by looking into ->leb_count too.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>