Add handling for first-time mtu exchange value

Partner reports too-small DLE lengths on first connection.  Analysis
suggests that this is due to a recent refactor falling back to the
(small) default MTU on a first connection rather than using the payload
size, which was the previous behavior -- on subsequent connections the
minimum of payload size or stored value will be used.

Cover this case.

Bug: 287911569
Test: atest bluetooth_test_gd_unit, net_test_stack_gatt, manual
Change-Id: Icaad8cfbab64b87f818f7bd7d999b5fc5d7a15b7