eatt: Fix missing tx mtu set

We use EATT only for LE and this fix incorrect value provided to the
EATT implementation as peer_mtu.

It could result on incorrect GATT fragmentation, when EATT was created
by remote device.

Tag: #feature
Test: atest BluetoothInstrumentationTests
Bug: 235783869
Change-Id: I4dde8eeb60ab0b045960a81718b428acbc0c34f7