Use L2CAP_DEFAULT_MTU if mtu is not specified

According to Bluetooth Core Spec 5.3 (Vol 3, Part A.5.1),
the MTU is asymmetric and the sender of the Request shall
specify the MTU it can receive on this channel if it differs
from the default value.

Bug: 215742460
Tag: #refactor
Test: atest CtsBluetoothTestCases bluetooth_test_gd_unit
Change-Id: Iceaba85ba7e812920a3c7d26980db02b58de0b8f