block/ps3vram: Remove obsolete reference to MTD

The ps3vram driver is a plain block device driver since commit
f507cd22 ("ps3/block: Replace mtd/ps3vram
by block/ps3vram").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>
Acked-by: Jim Paris <jim@jtan.com>
Signed-off-by: Jens Axboe <axboe@fb.com>