MTU handshake results in default size

[pperanich]

When running the ble_bas_peripheral example on the Microbit, the agreed upon MTU results in the BLE default of 23 bytes, instead of the requested 247 that is specified L2CAP_MTU - 4. When running a similar application with nrf-softdevice (with the same devices for gatt server/client), the MTU handshake results in the upper limit being selected, i.e. whatever ATT_MTU sets.