leaudio: Change required MTU

Note, still MTU 517 will be requested by GATT, but with this
change, DLE will stay on 64 octects.
This could help controllers as for now we set max on DLE.

Bug: 257250188
Test: manual
Change-Id: I18d3b51c6ebf5bafad9e5dfd5fad58b5fe659578