NFC: nci: Fix sparse: symbol 'nci_get_prop_rf_protocol' was not declared.

Fix sparse warning introduced by commit: 9e87f9a9

It was generating the following warning:
net/nfc/nci/ntf.c:170:7: sparse: symbol 'nci_get_prop_rf_protocol' was not declared. Should it be static?

Procedure to reproduce it:
# apt-get install sparse
  git checkout 9e87f9a9
  make ARCH=x86_64 allmodconfig
  make C=1 CF=-D__CHECK_ENDIAN__
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NChristophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>

net/nfc/nci/ntf.c

@@ -179,7 +179,7 @@ static __u8 *nci_extract_rf_params_nfcf_passive_listen(struct nci_dev *ndev,
 	return data;
 }
 
-__u32 nci_get_prop_rf_protocol(struct nci_dev *ndev, __u8 rf_protocol)
+static __u32 nci_get_prop_rf_protocol(struct nci_dev *ndev, __u8 rf_protocol)
 {
 	if (ndev->ops->get_rfprotocol)
 		return ndev->ops->get_rfprotocol(ndev, rf_protocol);