nfc: nci: hci: Add check on skb nci_hci_send_cmd parameter

skb can be NULL and may lead to a NULL pointer error.

Add a check condition before setting HCI rx buffer.

Cc: stable@vger.kernel.org
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>