nfc: st-nci: Free data with irrelevant NDLC PCB_SYNC value

PCB_SYNC different than PCB_TYPE_SUPERVISOR or PCB_TYPE_DATAFRAME
should be discarded.

Irrelevant data may be forwarded up to the ndlc state machine by
phys like spi to prevent missing potential data during "write"
transactions.

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