NFC: nci: Add reference to the RF logical connection

The NCI_STATIC_RF_CONN_ID logical connection is the most used
connection. Keeping it directly accessible in the nci_dev
structure will simplify and optimize the access.

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