Bluetooth: host: Fix not starting slave conn param update timer

When issuing LE Set Data Length Command host should not assume that
LE Data Length Change Event will be generated. From Core Spec 5.0:
"If the command causes the maximum transmission packet size or maximum
packet transmission time to change, an LE Data Length Change Event
shall be generated."

Change-Id: I17723b58ed4f390aa465db3f69126ee229871123
Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>