[MTD] Fix ssfdc blksize typo

I will not commit even trivial and obvious one-line fixes without building.
I will not commit even trivial and obvious one-line fixes without building.
I will not commit even trivial and obvious one-line fixes without building.
I will not commit even trivial and obvious one-line fixes without building.
Only clever people can get away with that.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>