Bluetooth: Refactor l2cap_retransmit_frame()

Make the code flow cleaner and changes the function to void.
It also fixes a potential NULL dereference with skb.

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>