Bluetooth: Fix drop of acked packets on ERTM

l2cap_drop_acked_frames() was droping not sent packets, causing them to
be not transmitted.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>