Bluetooth: Simplify l2cap_add_to_srej_queue

Make it easier to see what is loop break condition.
skb_queue_next return valid skb or garbage, not NULL.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>