NFC: st21nfca: ERR_PTR vs NULL fix

"skb" can be NULL here but it can't be an ERR_PTR:
- IS_ERR(NULL) return false and skb migth be NULL.
- skb cannot be a ERR_PTR as nfc_hci_send_cmd_async it never using such cast.

!skb is more appropriate at those places.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>