UBI attach in NOR flash may fail

[wang.bo116 at zte.com.cn]

Hello:
        Use UBI in nor flash, the mtd device's erase region may be 1,not 
0. In function io_init() when "ubi->mtd->numeraseregions != 0" will return 
EINVAL, i think chang the condition to "ubi->mtd->numeraseregions > 1" 
will better.