nfc: constify pointer to nfc_vendor_cmd

Neither the core nor the drivers modify the passed pointer to struct
nfc_vendor_cmd, so make it a pointer to const for correctness and
safety.

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