nfc: st-nci: Move loopback usage from HCI to NCI

NCI provides possible way to run loopback testing has done over HCI.

For us it offers many advantages:
- It simplifies the code: No more need for a vendor_cmds structure
- Loopback over HCI may not be supported in future st-nci firmware

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