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>