Bluetooth: L2CAP: Fix possible endless loop

cid is uint16_t and L2CAP_BR_CID_DYN_END is 0xffff so doing
"cid < L2CAP_BR_CID_DYN_END" comparisong is always true resulting
in for loop not being terminated as expected. Check against cid
overflow instead. Code comment is also added for clarity.

Change-Id: I15d6d838ed8b731824e602d089d765614c96c6c1
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>