vf610_nfc_probe() misses error handling of mtd_device_register().

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: NAlexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: NStefan Agner <stefan@agner.ch>
Signed-off-by: NBoris Brezillon <boris.brezillon@bootlin.com>