NFC: llcp: Fix usage of llcp_add_tlv()

NFC: llcp: Fix usage of llcp_add_tlv()

In functions using llcp_add_tlv(), a skb pointer could be set to NULL
and then reuse afterward.

With this patch, the skb pointer returned by llcp_add_tlv() is ignored
since it can only be the passed skb pointer or NULL when the passed TLV
is NULL. There is also no need to check for the TLV pointer as this is
done by llcp_add_tlv().

Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>