BLE Host - Clean up ACL tx fragmentation.
BLE Host - Clean up ACL tx fragmentation.

This commit does not result in a change in behavior.  The fragmentation
code was overly complicated.  This change just simplifies the
implementation.

X-Original-Commit: 009bb6cce4be2a2431d4084f630f152220abeb0d

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble