mkfs.ubifs: do not ignore --max-leb-cnt when formatting an UBI volume

When the output file is an UBI volume - mkfs.ubifs just sets --max-leb-cnt
to the volume size and ignores the user-supplied --max-leb-cnt value, which
is wrong. Let's set it to the volume size only if the user did not supply
--max-leb-cnt.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>