Bluetooth: L2CAP: Limit BR/EDR L2CAP RX MTU to configured value

Ensure that the BR/EDR L2CAP RX MTU is limited to the configured
BT_L2CAP_RX_MTU value during channel configuration.

This change prevent potential buffer overflow issues when receiving
data larger than the configured buffer size.

Signed-off-by: Jiawei Yang <jiawei.yang_1@nxp.com>