Bluetooth: Always allow SCO packets for user channel

The SCO packets from Bluetooth raw socket are now rejected because
hci_conn_num is left 0. This patch allows such the usecase to enable
the userspace SCO support.

Fixes: b16b327e ("Bluetooth: btusb: add sysfs attribute to control USB alt setting")
Signed-off-by: Hsin-chen Chuang <chharry@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>