Bluetooth: controller: llcp: fix PHY procedure for conformance test

This PR fixes the PHY update procedures for conformance tests when
being a Central
The problem was that data was in the LLL tx queue and was still being
queued before the PHY IND was queued (with a given instant).
As a result by the time the PHY IND was transmitted over the air the
instant was in the past.
The fix is to ensure that the LLL tx queue is empty, and to stop
queueing new data  before queueing the PHY IND

Following tests are fixed:
LL/CON/CEN/BV-49-C
LL/CON/CEN/BV-50-C
LL/CON/CEN/BV-53-C
LL/CON/CEN/BV-54-C

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>
(cherry picked from commit aa80f7da)