NFC: st21nfca: Fix logic when setting session_id

If dev_num >= ST21NFCA_NUM_DEVICES, the driver was returning an incorrect
success return code.

Once dev_num is set, it was not stated as busy.

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