Bluetooth: l2cap: Process valid commands in too long frame

This is required for passing PTS test cases:
- L2CAP/COS/CED/BI-14-C
  Multiple Signaling Command in one PDU, Data Truncated, BR/EDR,
  Connection Request
- L2CAP/COS/CED/BI-15-C
  Multiple Signaling Command in one PDU, Data Truncated, BR/EDR,
  Disconnection Request

The test procedure defined in L2CAP.TS.p39 for both tests is:
1. The Lower Tester sends a C-frame to the IUT with PDU Length set
   to 8 and Channel ID set to the correct signaling channel for the
   logical link. The Information payload contains one L2CAP_ECHO_REQ
   packet with Data Length set to 0 with 0 octets of echo data and
   one command packet and Data Length set as specified in Table 4.6
   and the correct command data.
2. The IUT sends an L2CAP_ECHO_RSP PDU to the Lower Tester.
3. Perform alternative 3A, 3B, 3C, or 3D depending on the IUT’s
   response.
   Alternative 3A (IUT terminates the link):
     3A.1 The IUT terminates the link.
     3A.2 The test ends with a Pass verdict.
   Alternative 3B (IUT discards the frame):
     3B.1 The IUT does not send a reply to the Lower Tester.
   Alternative 3C (IUT rejects PDU):
     3C.1 The IUT sends an L2CAP_COMMAND_REJECT_RSP PDU to the
          Lower Tester.
   Alternative 3D (Any other IUT response):
     3D.1 The Upper Tester issues a warning and the test ends.
4. The Lower Tester sends a C-frame to the IUT with PDU Length set
   to 4 and Channel ID set to the correct signaling channel for the
   logical link. The Information payload contains Data Length set to
   0 with an L2CAP_ECHO_REQ packet with 0 octets of echo data.
5. The IUT sends an L2CAP_ECHO_RSP PDU to the Lower Tester.

With expected outcome:
  In Steps 2 and 5, the IUT responds with an L2CAP_ECHO_RSP.
  In Step 3A.1, the IUT terminates the link.
  In Step 3B.1, the IUT does not send a reply to the Lower Tester.
  In Step 3C.1, the IUT rejects the PDU.
  In Step 3D.1, the IUT sends any valid response.

Currently PTS fails with the following logs:
  Failed to receive ECHO RESPONSE.

And HCI logs:
> ACL Data RX: Handle 11 flags 0x02 dlen 20
      L2CAP: Information Response (0x0b) ident 2 len 12
        Type: Fixed channels supported (0x0003)
        Result: Success (0x0000)
        Channels: 0x000000000000002e
          L2CAP Signaling (BR/EDR)
          Connectionless reception
          AMP Manager Protocol
          L2CAP Signaling (LE)
> ACL Data RX: Handle 11 flags 0x02 dlen 13
        frame too long
        08 01 00 00 08 02 01 00 aa                       .........

Cc: stable@vger.kernel.org
Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>