fs: yaffs2: switch to the generic xattr handler

The {get, set, remove}xattr inode operations have been already removed
in commit fd50ecad("vfs: Remove {get,set,remove}xattr inode operations").
Adjust the yaffs2 to use the generic xattr handler.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>