mtg: docg3: potential divide by zero in doc_write_oob()

If we set oobdelta to zero then we will either return -EINVAL or hit
a divide (modulus) by zero on the next line when we check
"(ooblen % oobdelta)".  It's better to just return -EINVAL here instead.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>