NFC: st21nfca: fix st21nfca_get_iso14443_3_uid data copy

st21nfca_get_iso14443_3_uid() does not correctly copy the uid from
uid_skb->data to its gate parameter. "gate = uid_skb->data;" only
puts a pointer to uid_skb->data to the local variable gate.
This means that in st21nfca_hci_target_from_gate() the content
of "u8 uid[NFC_NFCID1_MAXSIZE]" local variable is never initialized
before being used in memcpy(target->nfcid1, uid, len).

Fix this by replacing the local variable assignment with a memcpy.

This was found by compiling Linux with
"gcc -Wunused-but-set-parameter".

Acked-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>