mtd: Use mtd->name when registering nvmem device

With this patch, we use the mtd->name instead of concatenating the name
with '0'.

Fixes: c4dfa25a ("mtd: add support for reading MTD devices via the nvmem API")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>