Bluetooth: Classic: L2CAP: Disconn channel if proposed MTU is invalid

Disconnect the L2CAP channel connection if the proposed MTU is less
than min MTU or more than local supported MTU.

The main scenes are as follows.
If the proposed MTU is less than MIN MTU.
1. The channel connection of client and server is established,
2. Client/server sends channel config REQ (MTU=50),
3. Peer replies channel config RQP (Unaccepted/success with MTU=30),
4. The client/server will repeat step 3~4 if the RSP is unacceptable.

With the change applied, the local will disconnect the L2CAP channel
connection in step 3.

If the proposed MTU is more than local supported MTU.
1. The channel connection of client and server is established,
2. Client/server sends channel config REQ (MTU=50),
3. Peer replies channel config RQP (Unaccepted/success with MTU=80),
4. The client/server will repeat step 3~4 if the RSP is unacceptable.

With the change applied, the local will disconnect the L2CAP channel
connection in step 3.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>