Bluetooth: ISO: don't leak skb in ISO_CONT RX

For ISO_CONT RX, the data from skb is copied to conn->rx_skb, but the
skb is leaked.

Free skb after copying its data.

Fixes: ccf74f23 ("Bluetooth: Add BTPROTO_ISO socket type")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>