Bluetooth: Fix sending LE data over USB

Now that we have support for LE connections, before discarding a
frame we must check if there's a LE connection over that transport.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Ville Tervo <ville.tervo@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>