Bluetooth: Classic: L2CAP: Fix issue is received data len less than 2

If the received data length is less than 2 in none basic mode, the
L2CAP channel connection will be dropped.

Only check the received data length if the `SAR` is
`BT_L2CAP_CONTROL_SAR_START`.

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