Bluetooth: Fix missing retransmission action with RR(P=1)

The Bluetooth SIG Profile Tuning Suite Software uses the CSA1 spec
to run the L2CAP tests. The new 3.0 spec has a missing
Retransmit-I-Frames action when the Remote side is Busy.
We still start the retransmission timer if Remote is Busy and unacked
frames > 0. We do everything we did before this change plus the
Retransmission of I-frames.

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>