Handled invalid MTU Size in L2cap Process Commands

Use case: Execute the L2CAP test suite on Codenomicon tool

(1)From tool Codenomicon Defensics 11 menu Utilities > Bluetooth Scan,
paired Bluetooth between pc and phone.
(2)From Defensics 11 Settings ,choose paired phone Bluetooth to be
Target Bluetooth device address in Basic configuration.
(3)Run the test case " Security - l2cap ".
(4)When run to the number 28, 29 test case,check the phone. BT turn
off and turn on

Failure: BT crash observed due to invalid MTU packets from test tool

Root cause: L2CAP is not validating packets in L2CAP Process Commands

Fix: Fixed by sending reject CMD for invalid commands from tool

Test: code compilation.

Change-Id: I8c503ab77d8fca3728a84c3457d8115e31ee3a13