mtd: nand: vf610: check mtd_device_register() return code

vf610_nfc_probe() misses error handling of mtd_device_register().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>