nfc: netlink: Warning fix

When NFC_ATTR_VENDOR_DATA is not set, data_len is 0 and data is NULL.

Fixes the following warning:

net/nfc/netlink.c:1536:3: warning: 'data' may be used uninitialized
+in this function [-Wmaybe-uninitialized]
      return cmd->doit(dev, data, data_len);

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