GATT: Initiate L2CAP disconnect only when the channel is open

Use case:
Crash found during L2CAP disconnect from faulty BLE APP.

Steps:
Run SNS testing.

Failure:
Crash during L2CAP disc.

Root cause:
Faulty BLE application triggering GATT disconnection
which internally triggering L2CAP disconnection without checking
proper state of channel.

Fix:
- Initiate L2CAP disconnect only when the channel is open
- Validate LCB during disconnection from upper layers

Change-Id: Ic9d065f095feba659c861828732647e968c20db8