ENGR00279980 ubi: attach: do not return -EINVAL if the mtd->numeraseregions is 1

ENGR00279980 ubi: attach: do not return -EINVAL if the mtd->numeraseregions is 1

If the master mtd does not have any slave mtd partitions,
and its numeraseregions is one(only has one erease block), and
we attach the master mtd with : ubiattach -m 0 -d 0

We will meet the error:
-------------------------------------------------------
root@freescale ~$ ubiattach /dev/ubi_ctrl -m 0 -d 0
UBI: attaching mtd0 to ubi0
UBI error: io_init: multiple regions, not implemented
ubiattach: error!: cannot attach mtd0
           error 22 (Invalid argument)
-------------------------------------------------------

In fact, if there is only one "erase block", we should not
prevent the attach.

This patch fixes it.

Signed-off-by: Huang Shijie <b32955@freescale.com>