Bluetooth: host: L2CAP: Fix configuration parameters not be checked

Now in other mode rather than basic mode, configuration parameters
include MTU, Flush Time and QoS have been checked.
But in basic mode, they also need to be checked, so the code about this
should not be controlled by macro CONFIG_BT_L2CAP_RET_FC.

Signed-off-by: Cheng Chang <cheng.chang@nxp.com>