Bluetooth: Controller: Fix PHY update for unsupported PHY

Fix PHY update procedure to handle unsupported PHY requested
by peer central device. PHY update complete will not be
generated to Host, connection is maintained on the old
PHY and the Controller will not respond to PDUs received on
the unsupported PHY.

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