For this kind of warning stuff, it may be caused by some symbols removed from the kernel, that is to say, the version of kernel is not compatible with the yaffs2. I suggest you downloading the newest version from cvs and take a try.
If still not work at first try, then try to manually build it as an module, not compiling it into the kernel.
One thing I'm interesting is which flash/ICs you are using. Why use yaffs2?