btm: Improve set data length in the controller

With this patch btm keeps track on what TX octects has been suggested to
the controller and does not repeat requestes if not necessary.

This is important for the upcoming MTU improvements

Bug: 257250188
Bug: 239776509
Test: manual
Tag: #feature
Change-Id: I3fe8ed048cd68e25c7e99e5e963bb75cb8dc2767