nfc: constify payload argument in nci_send_cmd()

The nci_send_cmd() payload argument is passed directly to skb_put_data()
which already accepts a pointer to const, so make it const as well for
correctness and safety.

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