Bluetooth: Classic: L2CAP: Handle shorter data length of the request

The response L2CAP_COMMAND_REJECT_RSP(BT_L2CAP_REJ_NOT_UNDERSTOOD)
should be replied if the received L2CAP command is a invalid L2CAP
signaling command packets with extra data in the Information Payload.

Send L2CAP_COMMAND_REJECT_RSP(BT_L2CAP_REJ_NOT_UNDERSTOOD) if it a
small L2CAP signaling PDU or it has short length.

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