Bluetooth: Controller: llcp: Fix PHY update instant val if no PHY change

BT Core spec 5.3 Vol 6, Part B section 2.4.23 LL_PHY_UPDATE_IND says:
"If both the PHY_C_TO_P and PHY_P_TO_C fields are zero then there is no
Instant and the Instant field is reserved for future use."
Fields that are reserved for future shall be filled with zeros.

New LLCPs implementation didn't handle this case and set the instant
as if there were a PHY change. That caused qualification tests to fail.

The commit fixes the issue.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>