NFC: st21nfcb: Fix improper ndlc T2 management

T2 was never started when sending a command.
Start it when sending a command for the first attempt
and stop it once we receive the answer.

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