nfc: virtual_ncidev: constify pointer to nfc_dev

virtual_ncidev_ioctl() does not modify struct nfc_dev, so local variable
can be a pointer to const.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>