gatt/eatt: Fix qualification test cases

This patch fixes GATT/SR/GAW/BV-12-C and GATT/SR/GAW/BV-13-C.

Make sure to use proper mtu when preparing response on the EATT channel.

Bug: 235352514
Test: atest BluetoothInstrumentationTests
Test: PTS
Tag: #feature
Change-Id: I3af2577fe348200d55c9aa781959600b9f9118dc