Bluetooth: Remove double check for BT_CONNECTED

The same check is done just before call l2cap_streaming_send()

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>