NFC: st21nfca: Fix potential double kfree_skb error

skb is already freed in st21nfca_tx_work and was freed also in
st21nfca_im_send_psl_req.

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