NFC: st21nfca: Fix potential skb leaks in NFC-DEP code

After a unsuccessful call to nfc_hci_send_event the skb was not
freed and might lead to memory leak.

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