Bluetooth: controller: Fixes to pass DLE/PHY conformance tests

Added implementation to pause data PDU transmission during
PHY update procedure in order to comply to BT Spec. v5.1
Vol.6, Part B, Section 5.1.10.1 Packet transmit time
restrictions.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>