nfc: constify passed nfc_dev

The struct nfc_dev is not modified by nfc_get_drvdata() and
nfc_device_name() so it can be made a const.

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