yaffs: repair yaffs_get_mtd_device

The function yaffs_get_mtd_device use wrong function to retrieve
mtd_info structure (using yaffs_get_mtd_device itself will cause
dead loop).

Use get_mtd_device to do this.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>