fs: yaffs2: fix the prototype of function yaffs_rename()

The "rename2" of i_op has been renamed to "rename" in commit
2773bf00("fs: rename "rename2" i_op to "rename""). We do what we
have done in commit 1cd66c93("fs: make remaining filesystems use .rename2")
for yaffs2 according to this change.

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