NFC: st21nfca: Change nfcid3 generation

nfcid3 is based on sensf_res value. target->sensf is never NULL
as it is a table.
Check the sensf_res_len instead to make sure sensf_res is set or not.

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