NFC: st21nfca: Synchronize i2c Tx and Rx path

Stabilize communication by using a mutex.
This avoids running a write transaction during a read retry or a read
transaction during a write retry.

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