Bluetooth: Fix unset of RemoteBusy flag for L2CAP

RemoteBusy flag need to be unset before l2cap_ertm_send(), otherwise
l2cap_ertm_send() will return without sending packets because it checks
that flag before start sending.

Signed-off-by: Gustavo F. Padovan <gustavo@las.ic.unicamp.br>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>