On st21nfcb the irq line might be kept to active state because of other
interfaces activity. This may generate i2c read tentative resulting in
i2c NACK.

This fix will currently let NDLC upper layer to decide when it is relevent
to signal to the physical layer when the chip as muted.
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>